Java Knowledge series 7

Pepole who make a greate contribution on common libaraies deserve our respect.

Component(Widget) / STL / Container(Collection)

1. 合成不会强迫我们的程序设计进入继承的分级结构中has-a relationship。同时，合成显得更加灵活，因为可以动态选择一种类型（以及行为），而继承要求在编译期间准确地知道一种类型。
2. Java的工具（实用程序）库提供了一些“集合类”（亦称作“容器类”，但该术语已由AWT使用，所以这里仍采用“集合”这一称呼）。利用这些集合类，我们可以容纳乃至操纵自己的对象。Aggregate; container;
3. 有两方面的问题将数组与其他集合类型区分开来：效率efficiency 和类型type。
4. 对于Java来说，为保存和访问一系列对象（实际是对象的句柄 handle, or pointer in C++）数组，最有效的方法莫过于数组。数组实际代表一个简单的线性序列，它使得元素的访问速度非常快，但我们却要为这种速度付出代价：创建一个数组对象时，它的大小是固定的，而且不可在那个数组对象的“存在时间”内发生改变。可创建特定大小的一个数组，然后假如用光了存储空间，就再创建一个新数组，将所有句柄从旧数组移到新数组。这属于“矢量”（Vector）类的行为.
5. 在Java中，由于对数组和集合都要进行范围检查，所以对性能有一定的影响。
6. 这些类都涉及对对象的处理——好象它们没有特定的类型。换言之，它们将其当作Object类型处理（Object类型是Java中所有类的“根”类）。从某个角度看，这种处理方法是非常合理的：我们仅需构建一个集合，然后任何Java对象都可以进入那个集合（除基本数据类型外built-in type——可用Java的基本类型封装类将其作为常数置入集合，或者将其封装到自己的类内，作为可以变化的值使用）。这再一次反映了数组优于常规集合：创建一个数组时，可令其容纳一种特定的类型。这意味着可进行编译期类型检查，预防自己设置了错误的类型，或者错误指定了准备提取的类型。
7. 无论使用的数组属于什么类型，数组标识符实际都是指向真实对象的一个句柄。那些对象本身是在内存“堆”里创建的。堆对象既可“隐式”创建（即默认产生），亦可“显式”创建（即明确指定，用一个new表达式）。堆对象的一部分（实际是我们能访问的唯一字段或方法）是只读的length（长度）成员，它告诉我们那个数组对象里最多能容纳多少元素。
8. 集合类只能容纳对象句柄。但对一个数组，却既可令其直接容纳基本类型的数据，亦可容纳指向对象的句柄。利用象Integer、Double之类的“封装器”类，可将基本数据类型的值置入一个集合里。
9. 象C和C++这样的语言会使问题复杂化，因为我们不能返回一个数组，只能返回指向数组的一个指针。这样就非常麻烦，因为很难控制数组的“存在时间”，它很容易造成内存“漏洞”的出现。Java采用的是类似的方法，但我们能“返回一个数组”。当然，此时返回的实际仍是指向数组的指针。但在Java里，我们永远不必担心那个数组的是否可用——只要需要，它就会自动存在。而且垃圾收集器会在我们完成后自动将其清除。
10. 为容纳一组对象，最适宜的选择应当是数组。而且假如容纳的是一系列基本数据类型，更是必须采用数组。Have to use array when containing built-in type.
11. 当我们编写程序时，通常并不能确切地知道最终需要多少个对象。有些时候甚至想用更复杂的方式来保存对象。为解决这个问题，Java提供了四种类型的“集合类”：Vector（矢量）、BitSet（位集）、Stack（堆栈）以及Hashtable（散列表）。
12. 使用Java集合的“缺点”是在将对象置入一个集合时丢失了类型信息。之所以会发生这种情况，是由于当初编写集合时，那个集合的程序员根本不知道用户到底想把什么类型置入集合。若指示某个集合只允许特定的类型，会妨碍它成为一个“常规用途”的工具，为用户带来麻烦。为解决这个问题，集合实际容纳的是类型为Object的一些对象的句柄。这种类型当然代表Java中的所有对象，因为它是所有类的根。当然，也要注意这并不包括基本数据类型，因为它们并不是从“任何东西”继承来的。(It is solved with template in C++)这是一个很好的方案，只是不适用下述场合：
    • (1) 将一个对象句柄置入集合时，由于类型信息会被抛弃，所以任何类型的对象都可进入我们的集合——即便特别指示它只能容纳特定类型的对象。举个例子来说，虽然指示它只能容纳猫，但事实上任何人都可以把一条狗扔进来。
    • (2) 由于类型信息不复存在，所以集合能肯定的唯一事情就是自己容纳的是指向一个对象的句柄。正式使用它之前，必须对其进行造型，使其具有正确的类型。
13. 参数化类型 parameterization type这类问题并不是孤立的——我们许多时候都要在其他类型的基础上创建新类型。此时，在编译期间拥有特定的类型信息是非常有帮助的。这便是“参数化类型”的概念。在C++中，它由语言通过“模板Template” 获得了直接支持。至少，Java保留了关键字generic，期望有一天能够支持参数化类型。但我们现在无法确定这一天何时会来临。
14. 可利用“反复器”（Iterator）的概念达到这个目的。它可以是一个对象，作用是遍历一系列对象，并选择那个序列中的每个对象，同时不让客户程序员知道或关注那个序列的基础结构。此外，我们通常认为反复器是一种“轻量级”对象；也就是说，创建它只需付出极少的代价。但也正是由于这个原因，我们常发现反复器存在一些似乎很奇怪的限制。例如，有些反复器只能朝一个方向移动。Java的Enumeration（枚举，注释②）便是具有这些限制的一个反复器的例子。除下面这些外，不可再用它做其他任何事情：
    • (1) 用一个名为elements()的方法要求集合为我们提供一个Enumeration。我们首次调用它的nextElement()时 The de facto meaning is to move to next element and return the current element，这个Enumeration会返回序列中的第一个元素。
    • (2) 用nextElement()获得下一个对象。
    • (3) 用hasMoreElements()检查序列中是否还有更多的对象。
15. ②：“反复器”这个词在C++和OOP的其他地方是经常出现的，所以很难确定为什么Java的开发者采用了这样一个奇怪的名字。Java 1.2的集合库修正了这个问题以及其他许多问题。
16. BitSet实际是由“二进制位”构成的一个Vector。如果希望高效率地保存大量“开－关”信息，就应使用BitSet。它只有从尺寸的角度看才有意义；如果希望的高效率的访问，那么它的速度会比使用一些固有类型的数组慢一些。此外，BitSet的最小长度是一个长整数（Long）的长度：64位。这意味着假如我们准备保存比这更小的数据，如8位数据，那么BitSet就显得浪费了。所以最好创建自己的类，用它容纳自己的标志位。
17. Stack有时也可以称为“后入先出”（LIFO）集合。换言之，我们在堆栈里最后“压入”的东西将是以后第一个“弹出”的。和其他所有Java集合一样，我们压入和弹出的都是“对象”，所以必须对自己弹出的东西进行“造型”。一种很少见的做法是拒绝使用Vector作为一个Stack的基本构成元素，而是从Vector里“继承”一个Stack。这样一来，它就拥有了一个Vector的所有特征及行为，另外加上一些额外的Stack行为。很难判断出设计者到底是明确想这样做，还是属于一种固有的设计。
18. 这种“从一系列对象中选择”的概念亦可叫作一个“映射 map”、“字典 dictionary”或者“关联数组 relation array”。从概念上讲，它看起来象一个Vector，但却不是通过数字来查找对象，而是用另一个对象来查找它们！这通常都属于一个程序中的重要进程。
19. 这个概念具体反映到抽象类Dictionary身上。该类的接口是非常直观的size()告诉我们其中包含了多少元素；isEmpty()判断是否包含了元素（是则为true）；put(Object key, Object value)添加一个值（我们希望的东西），并将其同一个键关联起来（想用于搜索它的东西）；get(Object key)获得与某个键对应的值；而remove(Object Key)用于从列表中删除“键－值”对。还可以使用枚举技术：keys()产生对键的一个枚举（Enumeration）；而elements()产生对所有值的一个枚举。这便是一个Dictionary（字典）的全部。
20. 散列码可以获取对象中的信息，然后将其转换成那个对象“相对唯一”的整数（int）。所有对象都有一个散列码，而hashCode()是根类Object的一个方法。Hashtable获取对象的hashCode()，然后用它快速查找键。这样可使性能得到大幅度提升（④）。
21. 大家或许认为此时要做的全部事情就是正确地覆盖hashCode()。但这样做依然行不能，除非再做另一件事情：覆盖也属于Object一部分的equals()。当散列表试图判断我们的键是否等于表内的某个键时，就会用到这个方法。同样地，默认的Object.equals()只是简单地比较对象地址，了在散列表中将自己的类作为键使用，必须同时覆盖override hashCode() 和equals() befor objectize
22. 调用了一个名为getProperties()的static方法，用于获得一个特殊的Properties对象，对系统的某些特征进行描述。list()属于Properties的一个方法，可将内容发给我们选择的任何流式输出。也有一个save()方法，可用它将属性列表写入一个文件，以便日后用load()方法读取。尽管Properties类是从Hashtable继承的，但它也包含了一个散列表，用于容纳“默认”属性的列表。所以假如没有在主列表里找到一个属性，就会自动搜索默认属性。
23. JGL实现了许多功能，可满足对一个集合库的大多数常规需求，它与C++的模板机制非常相似。JGL包括相互链接起来的列表、设置、队列、映射、堆栈、序列以及反复器，它们的功能比Enumeration（枚举）强多了。同时提供了一套完整的算法，如检索和排序等。
24. JGL已包括到一些厂商发行的Java套件中，而且ObjectSpace公司自己也允许所有用户免费使用JGL，包括商业性的使用。详细情况和软件下载可访问http://www.ObjectSpace.com。与JGL配套提供的联机文档做得非常好，可作为自己的一个绝佳起点使用。
25. 我感觉特别好的一个是用“反复器”（Inerator）代替了“枚举”（Enumeration）。
26. 此次重新设计也加强了集合库的功能。现在新增的行为包括链接列表、队列以及撤消组队（即“双终点队列”）。集合库的设计是相当困难的（会遇到大量库设计问题）。在C++中，STL用多个不同的类来覆盖基础。这种做法比起STL以前是个很大的进步，那时根本没做这方面的考虑。但仍然没有很好地转换到Java里面。结果就是一大堆特别容易混淆的类。在另一个极端，我曾发现一个集合库由单个类构成：colleciton，它同时作为Vector和Hashtable使用。新集合库的设计者则希望达到一种新的平衡：实现人们希望从一个成熟集合库上获得的完整功能，同时又要比STL和其他类似的集合库更易学习和使用。这样得到的结果在某些场合显得有些古怪。但和早期Java库的一些决策不同，这些古怪之处并非偶然出现的，而是以复杂性作为代价，在进行仔细权衡之后得到的结果。这样做也许会延长人们掌握一些库概念的时间，但很快就会发现自己很乐于使用那些新工具，而且变得越来越离不了它。
27. 新的集合库考虑到了“容纳自己对象”的问题，并将其分割成两个明确的概念：
    • (1) 集合（Collection）：一组单独的元素，通常应用了某种规则。在这里，一个List（列表）必须按特定的顺序容纳元素，而一个Set（集）不可包含任何重复的元素。相反，“包”（Bag）的概念未在新的集合库中实现，因为“列表”已提供了类似的功能。
    • (2) 映射（Map）：一系列“键－值”对（这已在散列表身上得到了充分的体现）。从表面看，这似乎应该成为一个“键－值”对的“集合”，但假若试图按那种方式实现它，就会发现实现过程相当笨拙。这进一步证明了应该分离成单独的概念。另一方面，可以方便地查看Map的某个部分。只需创建一个集合，然后用它表示那一部分即可。这样一来，Map就可以返回自己键的一个Set、一个包含自己值的List或者包含自己“键－值”对的一个List。
28. 利用iterator()方法，所有集合都能生成一个“反复器”（Iterator）。反复器其实就象一个“枚举”（Enumeration），是后者的一个替代物，只是：
    • (1) 它采用了一个历史上默认、而且早在OOP中得到广泛采纳的名字（反复器）。
    • (2) 采用了比Enumeration更短的名字：hasNext()代替了hasMoreElement()，而next()代替了nextElement()。
    • (3) 添加了一个名为remove()的新方法，可删除由Iterator生成的上一个元素。所以每次调用next()的时候，只需调用remove()一次。在SimpleCollection.java中，大家可看到创建了一个反复器，并用它在集合里遍历，打印出每个元素。
29. 用一个集合能做的所有事情（亦可对Set和List做同样的事情，尽管List还提供了一些额外的功能）。Map不是从Collection继承的，所以要单独对待
30. Set拥有与Collection完全相同的接口，所以和两种不同的List不同，它没有什么额外的功能。相反，Set完全就是一个Collection，只是具有不同的行为（这是实例和多形性最理想的应用：用于表达不同的行为）。在这里，一个Set只允许每个对象存在一个实例（正如大家以后会看到的那样，一个对象的“值”的构成是相当复杂的）。
31. Set（接口） 添加到Set的每个元素都必须是独一无二的；否则Set就不会添加重复的元素。添加到Set里的对象必须定义equals()，从而建立对象的唯一性。Set拥有与Collection完全相同的接口。一个Set不能保证自己可按任何特定的顺序维持自己的元素
    • HashSet＊ 用于除非常小的以外的所有Set。对象也必须定义hashCode()
    • ArraySet 由一个数组后推得到的Set。面向非常小的Set设计，特别是那些需要频繁创建和删除的。对于小Set，与HashSet相比，ArraySet创建和反复所需付出的代价都要小得多。但随着Set的增大，它的性能也会大打折扣。不需要HashCode()
    • TreeSet 由一个“红黑树”后推得到的顺序Set（注释⑦）。这样一来，我们就可以从一个Set里提到一个顺序集合
32. Map（接口） 维持“键－值”对应关系（对），以便通过一个键查找相应的值
33. HashMap＊ 基于一个散列表实现（用它代替Hashtable）。针对“键－值”对的插入和检索，这种形式具有最稳定的性能。可通过构建器对这一性能进行调整，以便设置散列表的“能力”和“装载因子”ArrayMap 由一个ArrayList后推得到的Map。对反复的顺序提供了精确的控制。面向非常小的Map设计，特别是那些需要经常创建和删除的。对于非常小的Map，创建和反复所付出的代价要比HashMap低得多。但在Map变大以后，性能也会相应地大幅度降低TreeMap 在一个“红－黑”树的基础上实现。
34. 可在ArraySet以及HashSet间作出选择，具体取决于Set的大小（如果需要从一个Set中获得一个顺序列表，请用TreeSet；注释⑧）
35. 选择不同的Map实施方案时，注意Map的大小对于性能的影响是最大的.
36. Arrays类为所有基本数据类型的数组提供了一个过载的sort()和binarySearch()，它们亦可用于String和Object。
37. 可用与数组相同的形式排序和搜索一个列表（List）。用于排序和搜索列表的静态方法包含在类Collections中，但它们拥有与Arrays中差不多的签名：sort(List)用于对一个实现了Comparable的对象列表进行排序；binarySearch(List,Object)用于查找列表中的某个对象；sort(List,Comparator)利用一个“比较器”对一个列表进行排序；而binarySearch(List,Object,Comparator)则用于查找那个列表中的一个对象（注释⑨）。
38. Synchronized关键字是“多线程”机制一个非常重要的部分。
39. 数组包含了对象的数字化索引。它容纳的是一种已知类型的对象，所以在查找一个对象时，不必对结果进行造型处理。数组可以是多维的，而且能够容纳基本数据类型。但是，一旦把它创建好以后，大小便不能变化了。
40. Vector（矢量）也包含了对象的数字索引——可将数组和Vector想象成随机访问集合。当我们加入更多的元素时，Vector能够自动改变自身的大小。但Vector只能容纳对象的句柄，所以它不可包含基本数据类型；而且将一个对象句柄从集合中取出来的时候，必须对结果进行造型处理。
41. Hashtable（散列表）属于Dictionary（字典）的一种类型，是一种将对象（而不是数字）同其他对象关联到一起的方式。散列表也支持对对象的随机访问，事实上，它的整个设计方案都在突出访问的“高速度”。
42. Stack（堆栈）是一种“后入先出”（LIFO）的队列.

• Collection Interfaces - Represent different types of collections, such as sets, lists and maps. These interfaces form the basis of the framework.
• General-purpose Implementations - Primary implementations of the collection interfaces.
• Legacy Implementations - The collection classes from earlier releases, Vector and Hashtable, have been retrofitted to implement the collection interfaces.
• Special-purpose Implementations - Implementations designed for use in special situations. These implementations display nonstandard performance characteristics, usage restrictions, or behavior.
• Concurrent Implementations - Implementations designed for highly concurrent use.
• Wrapper Implementations - Add functionality, such as synchronization, to other implementations.
• Convenience Implementations - High-performance "mini-implementations" of the collection interfaces.
• Abstract Implementations - Partial implementations of the collection interfaces to facilitate custom implementations.
• Algorithms - Static methods that perform useful functions on collections, such as sorting a list.
• Infrastructure - Interfaces that provide essential support for the collection interfaces.
• Array Utilities - Utility functions for arrays of primitives and reference objects. Not, strictly speaking, a part of the Collections Framework, this functionality was added to the Java platform at the same time and relies on some of the same infrastructure.

Iterable Interface

import java.util.Iterator;
/**
 * Instances of classes that implement this interface can be used with
 * the enhanced for loop.
 *
 * @since 1.5
 */
public interface Iterable<T> {

    /**
     * Returns an {@link Iterator} for the elements in this object.
     *
     * @return An {@code Iterator} instance.
     */
    Iterator<T> iterator();
}

Collection Interface

package java.util;


/**
 * {@code Collection} is the root of the collection hierarchy. It defines operations on
 * data collections and the behavior that they will have in all implementations
 * of {@code Collection}s.
 *
 * All direct or indirect implementations of {@code Collection} should implement at
 * least two constructors. One with no parameters which creates an empty
 * collection and one with a parameter of type {@code Collection}. This second
 * constructor can be used to create a collection of different type as the
 * initial collection but with the same elements. Implementations of {@code Collection}
 * cannot be forced to implement these two constructors but at least all
 * implementations under {@code java.util} do.
 *
 * Methods that change the content of a collection throw an
 * {@code UnsupportedOperationException} if the underlying collection does not
 * support that operation, though it's not mandatory to throw such an {@code Exception}
 * in cases where the requested operation would not change the collection. In
 * these cases it's up to the implementation whether it throws an
 * {@code UnsupportedOperationException} or not.
 *
 * Methods marked with (optional) can throw an
 * {@code UnsupportedOperationException} if the underlying collection doesn't
 * support that method.
 */
public interface Collection<E> extends Iterable<E> {

    /**
     * Attempts to add {@code object} to the contents of this
     * {@code Collection} (optional).
     *
     * After this method finishes successfully it is guaranteed that the object
     * is contained in the collection.
     *
     * If the collection was modified it returns {@code true}, {@code false} if
     * no changes were made.
     *
     * An implementation of {@code Collection} may narrow the set of accepted
     * objects, but it has to specify this in the documentation. If the object
     * to be added does not meet this restriction, then an
     * {@code IllegalArgumentException} is thrown.
     *
     * If a collection does not yet contain an object that is to be added and
     * adding the object fails, this method <i>must</i> throw an appropriate
     * unchecked Exception. Returning false is not permitted in this case
     * because it would violate the postcondition that the element will be part
     * of the collection after this method finishes.
     *
     * @param object
     *            the object to add.
     * @return {@code true} if this {@code Collection} is
     *         modified, {@code false} otherwise.
     *
     * @throws UnsupportedOperationException
     *                if adding to this {@code Collection} is not supported.
     * @throws ClassCastException
     *                if the class of the object is inappropriate for this
     *                collection.
     * @throws IllegalArgumentException
     *                if the object cannot be added to this {@code Collection}.
     * @throws NullPointerException
     *                if null elements cannot be added to the {@code Collection}.
     */
    public boolean add(E object);

    /**
     * Attempts to add all of the objects contained in {@code Collection}
     * to the contents of this {@code Collection} (optional). If the passed {@code Collection}
     * is changed during the process of adding elements to this {@code Collection}, the
     * behavior is not defined.
     *
     * @param collection
     *            the {@code Collection} of objects.
     * @return {@code true} if this {@code Collection} is modified, {@code false}
     *         otherwise.
     * @throws UnsupportedOperationException
     *                if adding to this {@code Collection} is not supported.
     * @throws ClassCastException
     *                if the class of an object is inappropriate for this
     *                {@code Collection}.
     * @throws IllegalArgumentException
     *                if an object cannot be added to this {@code Collection}.
     * @throws NullPointerException
     *                if {@code collection} is {@code null}, or if it
     *                contains {@code null} elements and this {@code Collection} does
     *                not support such elements.
     */
    public boolean addAll(Collection<? extends E> collection);

    /**
     * Removes all elements from this {@code Collection}, leaving it empty (optional).
     *
     * @throws UnsupportedOperationException
     *                if removing from this {@code Collection} is not supported.
     *
     * @see #isEmpty
     * @see #size
     */
    public void clear();

    /**
     * Tests whether this {@code Collection} contains the specified object. Returns
     * {@code true} if and only if at least one element {@code elem} in this
     * {@code Collection} meets following requirement:
     * {@code (object==null ? elem==null : object.equals(elem))}.
     *
     * @param object
     *            the object to search for.
     * @return {@code true} if object is an element of this {@code Collection},
     *         {@code false} otherwise.
     * @throws ClassCastException
     *                if the object to look for isn't of the correct
     *                type.
     * @throws NullPointerException
     *                if the object to look for is {@code null} and this
     *                {@code Collection} doesn't support {@code null} elements.
     */
    public boolean contains(Object object);

    /**
     * Tests whether this {@code Collection} contains all objects contained in the
     * specified {@code Collection}. If an element {@code elem} is contained several
     * times in the specified {@code Collection}, the method returns {@code true} even
     * if {@code elem} is contained only once in this {@code Collection}.
     *
     * @param collection
     *            the collection of objects.
     * @return {@code true} if all objects in the specified {@code Collection} are
     *         elements of this {@code Collection}, {@code false} otherwise.
     * @throws ClassCastException
     *                if one or more elements of {@code collection} isn't of the
     *                correct type.
     * @throws NullPointerException
     *                if {@code collection} contains at least one {@code null}
     *                element and this {@code Collection} doesn't support {@code null}
     *                elements.
     * @throws NullPointerException
     *                if {@code collection} is {@code null}.
     */
    public boolean containsAll(Collection<?> collection);

    /**
     * Compares the argument to the receiver, and returns true if they represent
     * the <em>same</em> object using a class specific comparison.
     *
     * @param object
     *            the object to compare with this object.
     * @return {@code true} if the object is the same as this object and
     *         {@code false} if it is different from this object.
     * @see #hashCode
     */
    public boolean equals(Object object);

    /**
     * Returns an integer hash code for the receiver. Objects which are equal
     * return the same value for this method.
     *
     * @return the receiver's hash.
     *
     * @see #equals
     */
    public int hashCode();

    /**
     * Returns if this {@code Collection} contains no elements.
     *
     * @return {@code true} if this {@code Collection} has no elements, {@code false}
     *         otherwise.
     *
     * @see #size
     */
    public boolean isEmpty();

    /**
     * Returns an instance of {@link Iterator} that may be used to access the
     * objects contained by this {@code Collection}. The order in which the elements are
     * returned by the iterator is not defined. Only if the instance of the
     * {@code Collection} has a defined order the elements are returned in that order.
     *
     * @return an iterator for accessing the {@code Collection} contents.
     */
    public Iterator<E> iterator();

    /**
     * Removes one instance of the specified object from this {@code Collection} if one
     * is contained (optional). The element {@code elem} that is removed
     * complies with {@code (object==null ? elem==null : object.equals(elem)}.
     *
     * @param object
     *            the object to remove.
     * @return {@code true} if this {@code Collection} is modified, {@code false}
     *         otherwise.
     * @throws UnsupportedOperationException
     *                if removing from this {@code Collection} is not supported.
     * @throws ClassCastException
     *                if the object passed is not of the correct type.
     * @throws NullPointerException
     *                if {@code object} is {@code null} and this {@code Collection}
     *                doesn't support {@code null} elements.
     */
    public boolean remove(Object object);

    /**
     * Removes all occurrences in this {@code Collection} of each object in the
     * specified {@code Collection} (optional). After this method returns none of the
     * elements in the passed {@code Collection} can be found in this {@code Collection}
     * anymore.
     *
     * @param collection
     *            the collection of objects to remove.
     * @return {@code true} if this {@code Collection} is modified, {@code false}
     *         otherwise.
     *
     * @throws UnsupportedOperationException
     *                if removing from this {@code Collection} is not supported.
     * @throws ClassCastException
     *                if one or more elements of {@code collection}
     *                isn't of the correct type.
     * @throws NullPointerException
     *                if {@code collection} contains at least one
     *                {@code null} element and this {@code Collection} doesn't support
     *                {@code null} elements.
     * @throws NullPointerException
     *                if {@code collection} is {@code null}.
     */
    public boolean removeAll(Collection<?> collection);

    /**
     * Removes all objects from this {@code Collection} that are not also found in the
     * {@code Collection} passed (optional). After this method returns this {@code Collection}
     * will only contain elements that also can be found in the {@code Collection}
     * passed to this method.
     *
     * @param collection
     *            the collection of objects to retain.
     * @return {@code true} if this {@code Collection} is modified, {@code false}
     *         otherwise.
     * @throws UnsupportedOperationException
     *                if removing from this {@code Collection} is not supported.
     * @throws ClassCastException
     *                if one or more elements of {@code collection}
     *                isn't of the correct type.
     * @throws NullPointerException
     *                if {@code collection} contains at least one
     *                {@code null} element and this {@code Collection} doesn't support
     *                {@code null} elements.
     * @throws NullPointerException
     *                if {@code collection} is {@code null}.
     */
    public boolean retainAll(Collection<?> collection);

    /**
     * Returns a count of how many objects this {@code Collection} contains.
     *
     * @return how many objects this {@code Collection} contains, or Integer.MAX_VALUE
     *         if there are more than Integer.MAX_VALUE elements in this
     *         {@code Collection}.
     */
    public int size();

    /**
     * Returns a new array containing all elements contained in this {@code Collection}.
     *
     * If the implementation has ordered elements it will return the element
     * array in the same order as an iterator would return them.
     *
     * The array returned does not reflect any changes of the {@code Collection}. A new
     * array is created even if the underlying data structure is already an
     * array.
     *
     * @return an array of the elements from this {@code Collection}.
     */
    public Object[] toArray();

    /**
     * Returns an array containing all elements contained in this {@code Collection}. If
     * the specified array is large enough to hold the elements, the specified
     * array is used, otherwise an array of the same type is created. If the
     * specified array is used and is larger than this {@code Collection}, the array
     * element following the {@code Collection} elements is set to null.
     *
     * If the implementation has ordered elements it will return the element
     * array in the same order as an iterator would return them.
     *
     * {@code toArray(new Object[0])} behaves exactly the same way as
     * {@code toArray()} does.
     *
     * @param array
     *            the array.
     * @return an array of the elements from this {@code Collection}.
     *
     * @throws ArrayStoreException
     *                if the type of an element in this {@code Collection} cannot be
     *                stored in the type of the specified array.
     */
    public <T> T[] toArray(T[] array);
}

List Interface

package java.util;


/**
 * A {@code List} is a collection which maintains an ordering for its elements. Every
 * element in the {@code List} has an index. Each element can thus be accessed by its
 * index, with the first index being zero. Normally, {@code List}s allow duplicate
 * elements, as compared to Sets, where elements have to be unique.
 */
public interface List<E> extends Collection<E> {
    /**
     * Inserts the specified object into this {@code List} at the specified location.
     * The object is inserted before the current element at the specified
     * location. If the location is equal to the size of this {@code List}, the object
     * is added at the end. If the location is smaller than the size of this
     * {@code List}, then all elements beyond the specified location are moved by one
     * position towards the end of the {@code List}.
     *
     * @param location
     *            the index at which to insert.
     * @param object
     *            the object to add.
     * @throws UnsupportedOperationException
     *                if adding to this {@code List} is not supported.
     * @throws ClassCastException
     *                if the class of the object is inappropriate for this
     *                {@code List}.
     * @throws IllegalArgumentException
     *                if the object cannot be added to this {@code List}.
     * @throws IndexOutOfBoundsException
     *                if {@code location < 0 || location > size()}
     */
    public void add(int location, E object);

    /**
     * Adds the specified object at the end of this {@code List}.
     *
     * @param object
     *            the object to add.
     * @return always true.
     * @throws UnsupportedOperationException
     *                if adding to this {@code List} is not supported.
     * @throws ClassCastException
     *                if the class of the object is inappropriate for this
     *                {@code List}.
     * @throws IllegalArgumentException
     *                if the object cannot be added to this {@code List}.
     */
    public boolean add(E object);

    /**
     * Inserts the objects in the specified collection at the specified location
     * in this {@code List}. The objects are added in the order they are returned from
     * the collection's iterator.
     *
     * @param location
     *            the index at which to insert.
     * @param collection
     *            the collection of objects to be inserted.
     * @return true if this {@code List} has been modified through the insertion, false
     *         otherwise (i.e. if the passed collection was empty).
     * @throws UnsupportedOperationException
     *                if adding to this {@code List} is not supported.
     * @throws ClassCastException
     *                if the class of an object is inappropriate for this
     *                {@code List}.
     * @throws IllegalArgumentException
     *                if an object cannot be added to this {@code List}.
     * @throws IndexOutOfBoundsException
     *                if {@code location < 0 || > size()}
     */
    public boolean addAll(int location, Collection<? extends E> collection);

    /**
     * Adds the objects in the specified collection to the end of this {@code List}. The
     * objects are added in the order in which they are returned from the
     * collection's iterator.
     *
     * @param collection
     *            the collection of objects.
     * @return {@code true} if this {@code List} is modified, {@code false} otherwise
     *         (i.e. if the passed collection was empty).
     * @throws UnsupportedOperationException
     *                if adding to this {@code List} is not supported.
     * @throws ClassCastException
     *                if the class of an object is inappropriate for this
     *                {@code List}.
     * @throws IllegalArgumentException
     *                if an object cannot be added to this {@code List}.
     */
    public boolean addAll(Collection<? extends E> collection);

    /**
     * Removes all elements from this {@code List}, leaving it empty.
     *
     * @throws UnsupportedOperationException
     *                if removing from this {@code List} is not supported.
     * @see #isEmpty
     * @see #size
     */
    public void clear();

    /**
     * Tests whether this {@code List} contains the specified object.
     *
     * @param object
     *            the object to search for.
     * @return {@code true} if object is an element of this {@code List}, {@code false}
     *         otherwise
     */
    public boolean contains(Object object);

    /**
     * Tests whether this {@code List} contains all objects contained in the
     * specified collection.
     *
     * @param collection
     *            the collection of objects
     * @return {@code true} if all objects in the specified collection are
     *         elements of this {@code List}, {@code false} otherwise.
     */
    public boolean containsAll(Collection<?> collection);

    /**
     * Compares the given object with the {@code List}, and returns true if they
     * represent the <em>same</em> object using a class specific comparison. For
     * {@code List}s, this means that they contain the same elements in exactly the same
     * order.
     *
     * @param object
     *            the object to compare with this object.
     * @return boolean {@code true} if the object is the same as this object,
     *         and {@code false} if it is different from this object.
     * @see #hashCode
     */
    public boolean equals(Object object);

    /**
     * Returns the element at the specified location in this {@code List}.
     *
     * @param location
     *            the index of the element to return.
     * @return the element at the specified location.
     * @throws IndexOutOfBoundsException
     *                if {@code location < 0 || >= size()}
     */
    public E get(int location);

    /**
     * Returns the hash code for this {@code List}. It is calculated by taking each
     * element' hashcode and its position in the {@code List} into account.
     *
     * @return the hash code of the {@code List}.
     */
    public int hashCode();

    /**
     * Searches this {@code List} for the specified object and returns the index of the
     * first occurrence.
     *
     * @param object
     *            the object to search for.
     * @return the index of the first occurrence of the object or -1 if the
     *         object was not found.
     */
    public int indexOf(Object object);

    /**
     * Returns whether this {@code List} contains no elements.
     *
     * @return {@code true} if this {@code List} has no elements, {@code false}
     *         otherwise.
     * @see #size
     */
    public boolean isEmpty();

    /**
     * Returns an iterator on the elements of this {@code List}. The elements are
     * iterated in the same order as they occur in the {@code List}.
     *
     * @return an iterator on the elements of this {@code List}.
     * @see Iterator
     */
    public Iterator<E> iterator();

    /**
     * Searches this {@code List} for the specified object and returns the index of the
     * last occurrence.
     *
     * @param object
     *            the object to search for.
     * @return the index of the last occurrence of the object, or -1 if the
     *         object was not found.
     */
    public int lastIndexOf(Object object);

    /**
     * Returns a {@code List} iterator on the elements of this {@code List}. The elements are
     * iterated in the same order that they occur in the {@code List}.
     *
     * @return a {@code List} iterator on the elements of this {@code List}
     *
     * @see ListIterator
     */
    public ListIterator<E> listIterator();

    /**
     * Returns a list iterator on the elements of this {@code List}. The elements are
     * iterated in the same order as they occur in the {@code List}. The iteration
     * starts at the specified location.
     *
     * @param location
     *            the index at which to start the iteration.
     * @return a list iterator on the elements of this {@code List}.
     * @throws IndexOutOfBoundsException
     *                if {@code location < 0 || location > size()}
     * @see ListIterator
     */
    public ListIterator<E> listIterator(int location);

    /**
     * Removes the object at the specified location from this {@code List}.
     *
     * @param location
     *            the index of the object to remove.
     * @return the removed object.
     * @throws UnsupportedOperationException
     *                if removing from this {@code List} is not supported.
     * @throws IndexOutOfBoundsException
     *                if {@code location < 0 || >= size()}
     */
    public E remove(int location);

    /**
     * Removes the first occurrence of the specified object from this {@code List}.
     *
     * @param object
     *            the object to remove.
     * @return true if this {@code List} was modified by this operation, false
     *         otherwise.
     * @throws UnsupportedOperationException
     *                if removing from this {@code List} is not supported.
     */
    public boolean remove(Object object);

    /**
     * Removes all occurrences in this {@code List} of each object in the specified
     * collection.
     *
     * @param collection
     *            the collection of objects to remove.
     * @return {@code true} if this {@code List} is modified, {@code false} otherwise.
     * @throws UnsupportedOperationException
     *                if removing from this {@code List} is not supported.
     */
    public boolean removeAll(Collection<?> collection);

    /**
     * Removes all objects from this {@code List} that are not contained in the
     * specified collection.
     *
     * @param collection
     *            the collection of objects to retain.
     * @return {@code true} if this {@code List} is modified, {@code false} otherwise.
     * @throws UnsupportedOperationException
     *                if removing from this {@code List} is not supported.
     */
    public boolean retainAll(Collection<?> collection);

    /**
     * Replaces the element at the specified location in this {@code List} with the
     * specified object. This operation does not change the size of the {@code List}.
     *
     * @param location
     *            the index at which to put the specified object.
     * @param object
     *            the object to insert.
     * @return the previous element at the index.
     * @throws UnsupportedOperationException
     *                if replacing elements in this {@code List} is not supported.
     * @throws ClassCastException
     *                if the class of an object is inappropriate for this
     *                {@code List}.
     * @throws IllegalArgumentException
     *                if an object cannot be added to this {@code List}.
     * @throws IndexOutOfBoundsException
     *                if {@code location < 0 || >= size()}
     */
    public E set(int location, E object);

    /**
     * Returns the number of elements in this {@code List}.
     *
     * @return the number of elements in this {@code List}.
     */
    public int size();

    /**
     * Returns a {@code List} of the specified portion of this {@code List} from the given start
     * index to the end index minus one. The returned {@code List} is backed by this
     * {@code List} so changes to it are reflected by the other.
     *
     * @param start
     *            the index at which to start the sublist.
     * @param end
     *            the index one past the end of the sublist.
     * @return a list of a portion of this {@code List}.
     * @throws IndexOutOfBoundsException
     *                if {@code start < 0, start > end} or {@code end >
     *                size()}
     */
    public List<E> subList(int start, int end);

    /**
     * Returns an array containing all elements contained in this {@code List}.
     *
     * @return an array of the elements from this {@code List}.
     */
    public Object[] toArray();

    /**
     * Returns an array containing all elements contained in this {@code List}. If the
     * specified array is large enough to hold the elements, the specified array
     * is used, otherwise an array of the same type is created. If the specified
     * array is used and is larger than this {@code List}, the array element following
     * the collection elements is set to null.
     *
     * @param array
     *            the array.
     * @return an array of the elements from this {@code List}.
     * @throws ArrayStoreException
     *                if the type of an element in this {@code List} cannot be stored
     *                in the type of the specified array.
     */
    public <T> T[] toArray(T[] array);
}

Set Interface

package java.util;


/**
 * A {@code Set} is a data structure which does not allow duplicate elements.
 *
 * @since 1.2
 */
public interface Set<E> extends Collection<E> {

    /**
     * Adds the specified object to this set. The set is not modified if it
     * already contains the object.
     *
     * @param object
     *            the object to add.
     * @return {@code true} if this set is modified, {@code false} otherwise.
     * @throws UnsupportedOperationException
     *             when adding to this set is not supported.
     * @throws ClassCastException
     *             when the class of the object is inappropriate for this set.
     * @throws IllegalArgumentException
     *             when the object cannot be added to this set.
     */
    public boolean add(E object);

    /**
     * Adds the objects in the specified collection which do not exist yet in
     * this set.
     *
     * @param collection
     *            the collection of objects.
     * @return {@code true} if this set is modified, {@code false} otherwise.
     * @throws UnsupportedOperationException
     *             when adding to this set is not supported.
     * @throws ClassCastException
     *             when the class of an object is inappropriate for this set.
     * @throws IllegalArgumentException
     *             when an object cannot be added to this set.
     */
    public boolean addAll(Collection<? extends E> collection);

    /**
     * Removes all elements from this set, leaving it empty.
     *
     * @throws UnsupportedOperationException
     *             when removing from this set is not supported.
     * @see #isEmpty
     * @see #size
     */
    public void clear();

    /**
     * Searches this set for the specified object.
     *
     * @param object
     *            the object to search for.
     * @return {@code true} if object is an element of this set, {@code false}
     *         otherwise.
     */
    public boolean contains(Object object);

    /**
     * Searches this set for all objects in the specified collection.
     *
     * @param collection
     *            the collection of objects.
     * @return {@code true} if all objects in the specified collection are
     *         elements of this set, {@code false} otherwise.
     */
    public boolean containsAll(Collection<?> collection);

    /**
     * Compares the specified object to this set, and returns true if they
     * represent the <em>same</em> object using a class specific comparison.
     * Equality for a set means that both sets have the same size and the same
     * elements.
     *
     * @param object
     *            the object to compare with this object.
     * @return boolean {@code true} if the object is the same as this object,
     *         and {@code false} if it is different from this object.
     * @see #hashCode
     */
    public boolean equals(Object object);

    /**
     * Returns the hash code for this set. Two set which are equal must return
     * the same value.
     *
     * @return the hash code of this set.
     *
     * @see #equals
     */
    public int hashCode();

    /**
     * Returns true if this set has no elements.
     *
     * @return {@code true} if this set has no elements, {@code false}
     *         otherwise.
     * @see #size
     */
    public boolean isEmpty();

    /**
     * Returns an iterator on the elements of this set. The elements are
     * unordered.
     *
     * @return an iterator on the elements of this set.
     * @see Iterator
     */
    public Iterator<E> iterator();

    /**
     * Removes the specified object from this set.
     *
     * @param object
     *            the object to remove.
     * @return {@code true} if this set was modified, {@code false} otherwise.
     * @throws UnsupportedOperationException
     *             when removing from this set is not supported.
     */
    public boolean remove(Object object);

    /**
     * Removes all objects in the specified collection from this set.
     *
     * @param collection
     *            the collection of objects to remove.
     * @return {@code true} if this set was modified, {@code false} otherwise.
     * @throws UnsupportedOperationException
     *             when removing from this set is not supported.
     */
    public boolean removeAll(Collection<?> collection);

    /**
     * Removes all objects from this set that are not contained in the specified
     * collection.
     *
     * @param collection
     *            the collection of objects to retain.
     * @return {@code true} if this set was modified, {@code false} otherwise.
     * @throws UnsupportedOperationException
     *             when removing from this set is not supported.
     */
    public boolean retainAll(Collection<?> collection);

    /**
     * Returns the number of elements in this set.
     *
     * @return the number of elements in this set.
     */
    public int size();

    /**
     * Returns an array containing all elements contained in this set.
     *
     * @return an array of the elements from this set.
     */
    public Object[] toArray();

    /**
     * Returns an array containing all elements contained in this set. If the
     * specified array is large enough to hold the elements, the specified array
     * is used, otherwise an array of the same type is created. If the specified
     * array is used and is larger than this set, the array element following
     * the collection elements is set to null.
     *
     * @param array
     *            the array.
     * @return an array of the elements from this set.
     * @throws ArrayStoreException
     *             when the type of an element in this set cannot be stored in
     *             the type of the specified array.
     * @see Collection#toArray(Object[])
     */
    public <T> T[] toArray(T[] array);
}

Queue Interface

public interface Queue<E> extends Collection<E> {
    /**
     * Inserts the specified element into this queue if it is possible to do so
     * immediately without violating capacity restrictions, returning
     * <tt>true</tt> upon success and throwing an <tt>IllegalStateException</tt>
     * if no space is currently available.
     *
     * @param e the element to add
     * @return <tt>true</tt> (as specified by {@link Collection#add})
     * @throws IllegalStateException if the element cannot be added at this
     *         time due to capacity restrictions
     * @throws ClassCastException if the class of the specified element
     *         prevents it from being added to this queue
     * @throws NullPointerException if the specified element is null and
     *         this queue does not permit null elements
     * @throws IllegalArgumentException if some property of this element
     *         prevents it from being added to this queue
     */
    boolean add(E e);

    /**
     * Inserts the specified element into this queue if it is possible to do
     * so immediately without violating capacity restrictions.
     * When using a capacity-restricted queue, this method is generally
     * preferable to {@link #add}, which can fail to insert an element only
     * by throwing an exception.
     *
     * @param e the element to add
     * @return <tt>true</tt> if the element was added to this queue, else
     *         <tt>false</tt>
     * @throws ClassCastException if the class of the specified element
     *         prevents it from being added to this queue
     * @throws NullPointerException if the specified element is null and
     *         this queue does not permit null elements
     * @throws IllegalArgumentException if some property of this element
     *         prevents it from being added to this queue
     */
    boolean offer(E e);

    /**
     * Retrieves and removes the head of this queue.  This method differs
     * from {@link #poll poll} only in that it throws an exception if this
     * queue is empty.
     *
     * @return the head of this queue
     * @throws NoSuchElementException if this queue is empty
     */
    E remove();

    /**
     * Retrieves and removes the head of this queue,
     * or returns <tt>null</tt> if this queue is empty.
     *
     * @return the head of this queue, or <tt>null</tt> if this queue is empty
     */
    E poll();

    /**
     * Retrieves, but does not remove, the head of this queue.  This method
     * differs from {@link #peek peek} only in that it throws an exception
     * if this queue is empty.
     *
     * @return the head of this queue
     * @throws NoSuchElementException if this queue is empty
     */
    E element();

    /**
     * Retrieves, but does not remove, the head of this queue,
     * or returns <tt>null</tt> if this queue is empty.
     *
     * @return the head of this queue, or <tt>null</tt> if this queue is empty
     */
    E peek();
}

SortedSet Interface

package java.util;

public interface SortedSet<E> extends Set<E> { 
    public Comparator<? super E> comparator();
    public E first();
    public SortedSet<E> headSet(E end);  
    public E last();
    public SortedSet<E> subSet(E start, E end);
    public SortedSet<E> tailSet(E start);
}

NavigableSet Interface

public interface NavigableSet<E> extends SortedSet<E> {
    E lower(E e);
    E floor(E e);
    E ceiling(E e);
    E higher(E e);
    E pollFirst();
    E pollLast();
    Iterator<E> iterator();
    NavigableSet<E> descendingSet();
    Iterator<E> descendingIterator();
    NavigableSet<E> subSet(E fromElement, boolean fromInclusive,
                           E toElement,   boolean toInclusive);
    NavigableSet<E> headSet(E toElement, boolean inclusive);
    NavigableSet<E> tailSet(E fromElement, boolean inclusive);
    SortedSet<E> subSet(E fromElement, E toElement);
    SortedSet<E> headSet(E toElement);
    SortedSet<E> tailSet(E fromElement);
}

AbstractCollection Abstract Class

public abstract class AbstractCollection<E> implements Collection<E> {

    /**
     * Constructs a new instance of this AbstractCollection.
     */
    protected AbstractCollection() {
    }

    public boolean add(E object) {
        throw new UnsupportedOperationException();
    }

    /**
     * Attempts to add all of the objects contained in {@code collection}
     * to the contents of this {@code Collection} (optional). This implementation
     * iterates over the given {@code Collection} and calls {@code add} for each
     * element. If any of these calls return {@code true}, then {@code true} is
     * returned as result of this method call, {@code false} otherwise. If this
     * {@code Collection} does not support adding elements, an {@code
     * UnsupportedOperationException} is thrown.
     * <p>
     * If the passed {@code Collection} is changed during the process of adding elements
     * to this {@code Collection}, the behavior depends on the behavior of the passed
     * {@code Collection}.
     *
     * @param collection
     *            the collection of objects.
     * @return {@code true} if this {@code Collection} is modified, {@code false}
     *         otherwise.
     * @throws UnsupportedOperationException
     *                if adding to this {@code Collection} is not supported.
     * @throws ClassCastException
     *                if the class of an object is inappropriate for this
     *                {@code Collection}.
     * @throws IllegalArgumentException
     *                if an object cannot be added to this {@code Collection}.
     * @throws NullPointerException
     *                if {@code collection} is {@code null}, or if it contains
     *                {@code null} elements and this {@code Collection} does not support
     *                such elements.
     */
    public boolean addAll(Collection<? extends E> collection) {
        boolean result = false;
        Iterator<? extends E> it = collection.iterator();
        while (it.hasNext()) {
            if (add(it.next())) {
                result = true;
            }
        }
        return result;
    }

    /**
     * Removes all elements from this {@code Collection}, leaving it empty (optional).
     * This implementation iterates over this {@code Collection} and calls the {@code
     * remove} method on each element. If the iterator does not support removal
     * of elements, an {@code UnsupportedOperationException} is thrown.
     * <p>
     * Concrete implementations usually can clear a {@code Collection} more efficiently
     * and should therefore overwrite this method.
     *
     * @throws UnsupportedOperationException
     *                it the iterator does not support removing elements from
     *                this {@code Collection}
     * @see #iterator
     * @see #isEmpty
     * @see #size
     */
    public void clear() {
        Iterator<E> it = iterator();
        while (it.hasNext()) {
            it.next();
            it.remove();
        }
    }

    /**
     * Tests whether this {@code Collection} contains the specified object. This
     * implementation iterates over this {@code Collection} and tests, whether any
     * element is equal to the given object. If {@code object != null} then
     * {@code object.equals(e)} is called for each element {@code e} returned by
     * the iterator until the element is found. If {@code object == null} then
     * each element {@code e} returned by the iterator is compared with the test
     * {@code e == null}.
     *
     * @param object
     *            the object to search for.
     * @return {@code true} if object is an element of this {@code Collection}, {@code
     *         false} otherwise.
     * @throws ClassCastException
     *                if the object to look for isn't of the correct type.
     * @throws NullPointerException
     *                if the object to look for is {@code null} and this
     *                {@code Collection} doesn't support {@code null} elements.
     */
    public boolean contains(Object object) {
        Iterator<E> it = iterator();
        if (object != null) {
            while (it.hasNext()) {
                if (object.equals(it.next())) {
                    return true;
                }
            }
        } else {
            while (it.hasNext()) {
                if (it.next() == null) {
                    return true;
                }
            }
        }
        return false;
    }

    /**
     * Tests whether this {@code Collection} contains all objects contained in the
     * specified {@code Collection}. This implementation iterates over the specified
     * {@code Collection}. If one element returned by the iterator is not contained in
     * this {@code Collection}, then {@code false} is returned; {@code true} otherwise.
     *
     * @param collection
     *            the collection of objects.
     * @return {@code true} if all objects in the specified {@code Collection} are
     *         elements of this {@code Collection}, {@code false} otherwise.
     * @throws ClassCastException
     *                if one or more elements of {@code collection} isn't of the
     *                correct type.
     * @throws NullPointerException
     *                if {@code collection} contains at least one {@code null}
     *                element and this {@code Collection} doesn't support {@code null}
     *                elements.
     * @throws NullPointerException
     *                if {@code collection} is {@code null}.
     */
    public boolean containsAll(Collection<?> collection) {
        Iterator<?> it = collection.iterator();
        while (it.hasNext()) {
            if (!contains(it.next())) {
                return false;
            }
        }
        return true;
    }

    /**
     * Returns if this {@code Collection} contains no elements. This implementation
     * tests, whether {@code size} returns 0.
     *
     * @return {@code true} if this {@code Collection} has no elements, {@code false}
     *         otherwise.
     *
     * @see #size
     */
    public boolean isEmpty() {
        return size() == 0;
    }

    /**
     * Returns an instance of {@link Iterator} that may be used to access the
     * objects contained by this {@code Collection}. The order in which the elements are
     * returned by the {@link Iterator} is not defined unless the instance of the
     * {@code Collection} has a defined order.  In that case, the elements are returned in that order.
     * <p>
     * In this class this method is declared abstract and has to be implemented
     * by concrete {@code Collection} implementations.
     *
     * @return an iterator for accessing the {@code Collection} contents.
     */
    public abstract Iterator<E> iterator();

    /**
     * Removes one instance of the specified object from this {@code Collection} if one
     * is contained (optional). This implementation iterates over this
     * {@code Collection} and tests for each element {@code e} returned by the iterator,
     * whether {@code e} is equal to the given object. If {@code object != null}
     * then this test is performed using {@code object.equals(e)}, otherwise
     * using {@code object == null}. If an element equal to the given object is
     * found, then the {@code remove} method is called on the iterator and
     * {@code true} is returned, {@code false} otherwise. If the iterator does
     * not support removing elements, an {@code UnsupportedOperationException}
     * is thrown.
     *
     * @param object
     *            the object to remove.
     * @return {@code true} if this {@code Collection} is modified, {@code false}
     *         otherwise.
     * @throws UnsupportedOperationException
     *                if removing from this {@code Collection} is not supported.
     * @throws ClassCastException
     *                if the object passed is not of the correct type.
     * @throws NullPointerException
     *                if {@code object} is {@code null} and this {@code Collection}
     *                doesn't support {@code null} elements.
     */
    public boolean remove(Object object) {
        Iterator<?> it = iterator();
        if (object != null) {
            while (it.hasNext()) {
                if (object.equals(it.next())) {
                    it.remove();
                    return true;
                }
            }
        } else {
            while (it.hasNext()) {
                if (it.next() == null) {
                    it.remove();
                    return true;
                }
            }
        }
        return false;
    }

    /**
     * Removes all occurrences in this {@code Collection} of each object in the
     * specified {@code Collection} (optional). After this method returns none of the
     * elements in the passed {@code Collection} can be found in this {@code Collection}
     * anymore.
     * <p>
     * This implementation iterates over this {@code Collection} and tests for each
     * element {@code e} returned by the iterator, whether it is contained in
     * the specified {@code Collection}. If this test is positive, then the {@code
     * remove} method is called on the iterator. If the iterator does not
     * support removing elements, an {@code UnsupportedOperationException} is
     * thrown.
     *
     * @param collection
     *            the collection of objects to remove.
     * @return {@code true} if this {@code Collection} is modified, {@code false}
     *         otherwise.
     * @throws UnsupportedOperationException
     *                if removing from this {@code Collection} is not supported.
     * @throws ClassCastException
     *                if one or more elements of {@code collection} isn't of the
     *                correct type.
     * @throws NullPointerException
     *                if {@code collection} contains at least one {@code null}
     *                element and this {@code Collection} doesn't support {@code null}
     *                elements.
     * @throws NullPointerException
     *                if {@code collection} is {@code null}.
     */
    public boolean removeAll(Collection<?> collection) {
        boolean result = false;
        Iterator<?> it = iterator();
        while (it.hasNext()) {
            if (collection.contains(it.next())) {
                it.remove();
                result = true;
            }
        }
        return result;
    }

    /**
     * Removes all objects from this {@code Collection} that are not also found in the
     * {@code Collection} passed (optional). After this method returns this {@code Collection}
     * will only contain elements that also can be found in the {@code Collection}
     * passed to this method.
     * <p>
     * This implementation iterates over this {@code Collection} and tests for each
     * element {@code e} returned by the iterator, whether it is contained in
     * the specified {@code Collection}. If this test is negative, then the {@code
     * remove} method is called on the iterator. If the iterator does not
     * support removing elements, an {@code UnsupportedOperationException} is
     * thrown.
     *
     * @param collection
     *            the collection of objects to retain.
     * @return {@code true} if this {@code Collection} is modified, {@code false}
     *         otherwise.
     * @throws UnsupportedOperationException
     *                if removing from this {@code Collection} is not supported.
     * @throws ClassCastException
     *                if one or more elements of {@code collection}
     *                isn't of the correct type.
     * @throws NullPointerException
     *                if {@code collection} contains at least one
     *                {@code null} element and this {@code Collection} doesn't support
     *                {@code null} elements.
     * @throws NullPointerException
     *                if {@code collection} is {@code null}.
     */
    public boolean retainAll(Collection<?> collection) {
        boolean result = false;
        Iterator<?> it = iterator();
        while (it.hasNext()) {
            if (!collection.contains(it.next())) {
                it.remove();
                result = true;
            }
        }
        return result;
    }

    /**
     * Returns a count of how many objects this {@code Collection} contains.
     * <p>
     * In this class this method is declared abstract and has to be implemented
     * by concrete {@code Collection} implementations.
     *
     * @return how many objects this {@code Collection} contains, or {@code Integer.MAX_VALUE}
     *         if there are more than {@code Integer.MAX_VALUE} elements in this
     *         {@code Collection}.
     */
    public abstract int size();

    public Object[] toArray() {
        int size = size(), index = 0;
        Iterator<?> it = iterator();
        Object[] array = new Object[size];
        while (index < size) {
            array[index++] = it.next();
        }
        return array;
    }

    @SuppressWarnings("unchecked")
    public <T> T[] toArray(T[] contents) {
        int size = size(), index = 0;
        if (size > contents.length) {
            Class<?> ct = contents.getClass().getComponentType();
            contents = (T[]) Array.newInstance(ct, size);
        }
        for (E entry : this) {
            contents[index++] = (T) entry;
        }
        if (index < contents.length) {
            contents[index] = null;
        }
        return contents;
    }

    /**
     * Returns the string representation of this {@code Collection}. The presentation
     * has a specific format. It is enclosed by square brackets ("[]"). Elements
     * are separated by ', ' (comma and space).
     *
     * @return the string representation of this {@code Collection}.
     */
    @Override
    public String toString() {
        if (isEmpty()) {
            return "[]";
        }

        StringBuilder buffer = new StringBuilder(size() * 16);
        buffer.append('[');
        Iterator<?> it = iterator();
        while (it.hasNext()) {
            Object next = it.next();
            if (next != this) {
                buffer.append(next);
            } else {
                buffer.append("(this Collection)");
            }
            if (it.hasNext()) {
                buffer.append(", ");
            }
        }
        buffer.append(']');
        return buffer.toString();
    }
}

 AbstractSet AbstractClass

public abstract class AbstractSet<E> extends AbstractCollection<E> implements
        Set<E> {

    /**
     * Constructs a new instance of this AbstractSet.
     */
    protected AbstractSet() {
    }

    /**
     * Compares the specified object to this Set and returns true if they are
     * equal. The object must be an instance of Set and contain the same
     * objects.
     *
     * @param object
     *            the object to compare with this set.
     * @return {@code true} if the specified object is equal to this set,
     *         {@code false} otherwise
     * @see #hashCode
     */
    @Override
    public boolean equals(Object object) {
        if (this == object) {
            return true;
        }
        if (object instanceof Set) {
            Set<?> s = (Set<?>) object;

            try {
                return size() == s.size() && containsAll(s);
            } catch (NullPointerException ignored) {
                return false;
            } catch (ClassCastException ignored) {
                return false;
            }
        }
        return false;
    }

    /**
     * Returns the hash code for this set. Two set which are equal must return
     * the same value. This implementation calculates the hash code by adding
     * each element's hash code.
     *
     * @return the hash code of this set.
     * @see #equals
     */
    @Override
    public int hashCode() {
        int result = 0;
        Iterator<?> it = iterator();
        while (it.hasNext()) {
            Object next = it.next();
            result += next == null ? 0 : next.hashCode();
        }
        return result;
    }

    /**
     * Removes all occurrences in this collection which are contained in the
     * specified collection.
     *
     * @param collection
     *            the collection of objects to remove.
     * @return {@code true} if this collection was modified, {@code false}
     *         otherwise.
     * @throws UnsupportedOperationException
     *                if removing from this collection is not supported.
     */
    @Override
    public boolean removeAll(Collection<?> collection) {
        boolean result = false;
        if (size() <= collection.size()) {
            Iterator<?> it = iterator();
            while (it.hasNext()) {
                if (collection.contains(it.next())) {
                    it.remove();
                    result = true;
                }
            }
        } else {
            Iterator<?> it = collection.iterator();
            while (it.hasNext()) {
                result = remove(it.next()) || result;
            }
        }
        return result;
    }
}

HashSet 

package java.util;

import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;


public class HashSet<E> extends AbstractSet<E> implements Set<E>, Cloneable,
        Serializable {

    private static final long serialVersionUID = -5024744406713321676L;

    transient HashMap<E, HashSet<E>> backingMap;

    /**
     * Constructs a new empty instance of {@code HashSet}.
     */
    public HashSet() {
        this(new HashMap<E, HashSet<E>>());
    }

    /**
     * Constructs a new instance of {@code HashSet} with the specified capacity.
     *
     * @param capacity
     *            the initial capacity of this {@code HashSet}.
     */
    public HashSet(int capacity) {
        this(new HashMap<E, HashSet<E>>(capacity));
    }

    /**
     * Constructs a new instance of {@code HashSet} with the specified capacity
     * and load factor.
     *
     * @param capacity
     *            the initial capacity.
     * @param loadFactor
     *            the initial load factor.
     */
    public HashSet(int capacity, float loadFactor) {
        this(new HashMap<E, HashSet<E>>(capacity, loadFactor));
    }

    /**
     * Constructs a new instance of {@code HashSet} containing the unique
     * elements in the specified collection.
     *
     * @param collection
     *            the collection of elements to add.
     */
    public HashSet(Collection<? extends E> collection) {
        this(new HashMap<E, HashSet<E>>(collection.size() < 6 ? 11 : collection
                .size() * 2));
        for (E e : collection) {
            add(e);
        }
    }

    HashSet(HashMap<E, HashSet<E>> backingMap) {
        this.backingMap = backingMap;
    }

    /**
     * Adds the specified object to this {@code HashSet} if not already present.
     *
     * @param object
     *            the object to add.
     * @return {@code true} when this {@code HashSet} did not already contain
     *         the object, {@code false} otherwise
     */
    @Override
    public boolean add(E object) {
        return backingMap.put(object, this) == null;
    }

    /**
     * Removes all elements from this {@code HashSet}, leaving it empty.
     *
     * @see #isEmpty
     * @see #size
     */
    @Override
    public void clear() {
        backingMap.clear();
    }

    /**
     * Returns a new {@code HashSet} with the same elements and size as this
     * {@code HashSet}.
     *
     * @return a shallow copy of this {@code HashSet}.
     * @see java.lang.Cloneable
     */
    @Override
    @SuppressWarnings("unchecked")
    public Object clone() {
        try {
            HashSet<E> clone = (HashSet<E>) super.clone();
            clone.backingMap = (HashMap<E, HashSet<E>>) backingMap.clone();
            return clone;
        } catch (CloneNotSupportedException e) {
            throw new AssertionError(e);
        }
    }

    /**
     * Searches this {@code HashSet} for the specified object.
     *
     * @param object
     *            the object to search for.
     * @return {@code true} if {@code object} is an element of this
     *         {@code HashSet}, {@code false} otherwise.
     */
    @Override
    public boolean contains(Object object) {
        return backingMap.containsKey(object);
    }

    /**
     * Returns true if this {@code HashSet} has no elements, false otherwise.
     *
     * @return {@code true} if this {@code HashSet} has no elements,
     *         {@code false} otherwise.
     * @see #size
     */
    @Override
    public boolean isEmpty() {
        return backingMap.isEmpty();
    }

    /**
     * Returns an Iterator on the elements of this {@code HashSet}.
     *
     * @return an Iterator on the elements of this {@code HashSet}.
     * @see Iterator
     */
    @Override
    public Iterator<E> iterator() {
        return backingMap.keySet().iterator();
    }

    /**
     * Removes the specified object from this {@code HashSet}.
     *
     * @param object
     *            the object to remove.
     * @return {@code true} if the object was removed, {@code false} otherwise.
     */
    @Override
    public boolean remove(Object object) {
        return backingMap.remove(object) != null;
    }

    /**
     * Returns the number of elements in this {@code HashSet}.
     *
     * @return the number of elements in this {@code HashSet}.
     */
    @Override
    public int size() {
        return backingMap.size();
    }

    private void writeObject(ObjectOutputStream stream) throws IOException {
        stream.defaultWriteObject();
        stream.writeInt(backingMap.table.length);
        stream.writeFloat(HashMap.DEFAULT_LOAD_FACTOR);
        stream.writeInt(size());
        for (E e : this) {
            stream.writeObject(e);
        }
    }

    @SuppressWarnings("unchecked")
    private void readObject(ObjectInputStream stream) throws IOException,
            ClassNotFoundException {
        stream.defaultReadObject();
        int length = stream.readInt();
        float loadFactor = stream.readFloat();
        backingMap = createBackingMap(length, loadFactor);
        int elementCount = stream.readInt();
        for (int i = elementCount; --i >= 0;) {
            E key = (E) stream.readObject();
            backingMap.put(key, this);
        }
    }

    HashMap<E, HashSet<E>> createBackingMap(int capacity, float loadFactor) {
        return new HashMap<E, HashSet<E>>(capacity, loadFactor);
    }
}

TreeSet

/*
 *  Licensed to the Apache Software Foundation (ASF) under one or more
 *  contributor license agreements.  See the NOTICE file distributed with
 *  this work for additional information regarding copyright ownership.
 *  The ASF licenses this file to You under the Apache License, Version 2.0
 *  (the "License"); you may not use this file except in compliance with
 *  the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 *  Unless required by applicable law or agreed to in writing, software
 *  distributed under the License is distributed on an "AS IS" BASIS,
 *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *  See the License for the specific language governing permissions and
 *  limitations under the License.
 */

package java.util;

import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;

/**
 * TreeSet is an implementation of SortedSet. All optional operations (adding
 * and removing) are supported. The elements can be any objects which are
 * comparable to each other either using their natural order or a specified
 * Comparator.
 *
 * @since 1.2
 */
public class TreeSet<E> extends AbstractSet<E> implements NavigableSet<E>,
        Cloneable, Serializable {

    private static final long serialVersionUID = -2479143000061671589L;

    /** Keys are this set's elements. Values are always Boolean.TRUE */
    private transient NavigableMap<E, Object> backingMap;

    private transient NavigableSet<E> descendingSet;

    TreeSet(NavigableMap<E, Object> map) {
        backingMap = map;
    }

    /**
     * Constructs a new empty instance of {@code TreeSet} which uses natural
     * ordering.
     */
    public TreeSet() {
        backingMap = new TreeMap<E, Object>();
    }

    /**
     * Constructs a new instance of {@code TreeSet} which uses natural ordering
     * and containing the unique elements in the specified collection.
     *
     * @param collection
     *            the collection of elements to add.
     * @throws ClassCastException
     *                when an element in the collection does not implement the
     *                Comparable interface, or the elements in the collection
     *                cannot be compared.
     */
    public TreeSet(Collection<? extends E> collection) {
        this();
        addAll(collection);
    }

    /**
     * Constructs a new empty instance of {@code TreeSet} which uses the
     * specified comparator.
     *
     * @param comparator
     *            the comparator to use.
     */
    public TreeSet(Comparator<? super E> comparator) {
        backingMap = new TreeMap<E, Object>(comparator);
    }

    /**
     * Constructs a new instance of {@code TreeSet} containing the elements of
     * the specified SortedSet and using the same Comparator.
     *
     * @param set
     *            the SortedSet of elements to add.
     */
    public TreeSet(SortedSet<E> set) {
        this(set.comparator());
        Iterator<E> it = set.iterator();
        while (it.hasNext()) {
            add(it.next());
        }
    }

    /**
     * Adds the specified object to this {@code TreeSet}.
     *
     * @param object
     *            the object to add.
     * @return {@code true} when this {@code TreeSet} did not already contain
     *         the object, {@code false} otherwise.
     * @throws ClassCastException
     *             when the object cannot be compared with the elements in this
     *             {@code TreeSet}.
     * @throws NullPointerException
     *             when the object is null and the comparator cannot handle
     *             null.
     */
    @Override
    public boolean add(E object) {
        return backingMap.put(object, Boolean.TRUE) == null;
    }

    /**
     * Adds the objects in the specified collection to this {@code TreeSet}.
     *
     * @param collection
     *            the collection of objects to add.
     * @return {@code true} if this {@code TreeSet} was modified, {@code false}
     *         otherwise.
     * @throws ClassCastException
     *             when an object in the collection cannot be compared with the
     *             elements in this {@code TreeSet}.
     * @throws NullPointerException
     *             when an object in the collection is null and the comparator
     *             cannot handle null.
     */
    @Override
    public boolean addAll(Collection<? extends E> collection) {
        return super.addAll(collection);
    }

    /**
     * Removes all elements from this {@code TreeSet}, leaving it empty.
     *
     * @see #isEmpty
     * @see #size
     */
    @Override
    public void clear() {
        backingMap.clear();
    }

    /**
     * Returns a new {@code TreeSet} with the same elements, size and comparator
     * as this {@code TreeSet}.
     *
     * @return a shallow copy of this {@code TreeSet}.
     * @see java.lang.Cloneable
     */
    @SuppressWarnings("unchecked")
    @Override
    public Object clone() {
        try {
            TreeSet<E> clone = (TreeSet<E>) super.clone();
            if (backingMap instanceof TreeMap) {
                clone.backingMap = (NavigableMap<E, Object>) ((TreeMap<E, Object>) backingMap)
                        .clone();
            } else {
                clone.backingMap = new TreeMap<E, Object>(backingMap);
            }
            return clone;
        } catch (CloneNotSupportedException e) {
            throw new AssertionError(e);
        }
    }

    /**
     * Returns the comparator used to compare elements in this {@code TreeSet}.
     *
     * @return a Comparator or null if the natural ordering is used
     */
    public Comparator<? super E> comparator() {
        return backingMap.comparator();
    }

    /**
     * Searches this {@code TreeSet} for the specified object.
     *
     * @param object
     *            the object to search for.
     * @return {@code true} if {@code object} is an element of this
     *         {@code TreeSet}, {@code false} otherwise.
     * @throws ClassCastException
     *             when the object cannot be compared with the elements in this
     *             {@code TreeSet}.
     * @throws NullPointerException
     *             when the object is null and the comparator cannot handle
     *             null.
     */
    @Override
    public boolean contains(Object object) {
        return backingMap.containsKey(object);
    }

    /**
     * Returns true if this {@code TreeSet} has no element, otherwise false.
     *
     * @return true if this {@code TreeSet} has no element.
     * @see #size
     */
    @Override
    public boolean isEmpty() {
        return backingMap.isEmpty();
    }

    /**
     * Returns an Iterator on the elements of this {@code TreeSet}.
     *
     * @return an Iterator on the elements of this {@code TreeSet}.
     * @see Iterator
     */
    @Override
    public Iterator<E> iterator() {
        return backingMap.keySet().iterator();
    }

    /**
     * {@inheritDoc}
     *
     * @see java.util.NavigableSet#descendingIterator()
     * @since 1.6
     */
    public Iterator<E> descendingIterator() {
        return descendingSet().iterator();
    }

    /**
     * Removes an occurrence of the specified object from this {@code TreeSet}.
     *
     * @param object
     *            the object to remove.
     * @return {@code true} if this {@code TreeSet} was modified, {@code false}
     *         otherwise.
     * @throws ClassCastException
     *             when the object cannot be compared with the elements in this
     *             {@code TreeSet}.
     * @throws NullPointerException
     *             when the object is null and the comparator cannot handle
     *             null.
     */
    @Override
    public boolean remove(Object object) {
        return backingMap.remove(object) != null;
    }

    /**
     * Returns the number of elements in this {@code TreeSet}.
     *
     * @return the number of elements in this {@code TreeSet}.
     */
    @Override
    public int size() {
        return backingMap.size();
    }

    /**
     * Returns the first element in this set.
     * @exception NoSuchElementException when this TreeSet is empty
     */
    public E first() {
        return backingMap.firstKey();
    }

    /**
     * Returns the last element in this set.
     * @exception NoSuchElementException when this TreeSet is empty
     */
    public E last() {
        return backingMap.lastKey();
    }

    /**
     * {@inheritDoc}
     *
     * @see java.util.NavigableSet#pollFirst()
     * @since 1.6
     */
    public E pollFirst() {
        Map.Entry<E, Object> entry = backingMap.pollFirstEntry();
        return (entry == null) ? null : entry.getKey();
    }

    /**
     * {@inheritDoc}
     *
     * @see java.util.NavigableSet#pollLast()
     * @since 1.6
     */
    public E pollLast() {
        Map.Entry<E, Object> entry = backingMap.pollLastEntry();
        return (entry == null) ? null : entry.getKey();
    }

    /**
     * {@inheritDoc}
     *
     * @see java.util.NavigableSet#higher(java.lang.Object)
     * @since 1.6
     */
    public E higher(E e) {
        return backingMap.higherKey(e);
    }

    /**
     * {@inheritDoc}
     *
     * @see java.util.NavigableSet#lower(java.lang.Object)
     * @since 1.6
     */
    public E lower(E e) {
        return backingMap.lowerKey(e);
    }

    /**
     * {@inheritDoc}
     *
     * @see java.util.NavigableSet#ceiling(java.lang.Object)
     * @since 1.6
     */
    public E ceiling(E e) {
        return backingMap.ceilingKey(e);
    }

    /**
     * {@inheritDoc}
     *
     * @see java.util.NavigableSet#floor(java.lang.Object)
     * @since 1.6
     */
    public E floor(E e) {
        return backingMap.floorKey(e);
    }

    /**
     * {@inheritDoc}
     *
     * @see java.util.NavigableSet#descendingSet()
     * @since 1.6
     */
    public NavigableSet<E> descendingSet() {
        return (descendingSet != null) ? descendingSet
                : (descendingSet = new TreeSet<E>(backingMap.descendingMap()));
    }

    /**
     * {@inheritDoc}
     *
     * @see java.util.NavigableSet#subSet(Object, boolean, Object, boolean)
     * @since 1.6
     */
    @SuppressWarnings("unchecked")
    public NavigableSet<E> subSet(E start, boolean startInclusive, E end,
            boolean endInclusive) {
        Comparator<? super E> c = backingMap.comparator();
        int compare = (c == null) ? ((Comparable<E>) start).compareTo(end) : c
                .compare(start, end);
        if (compare <= 0) {
            return new TreeSet<E>(backingMap.subMap(start, startInclusive, end,
                    endInclusive));
        }
        throw new IllegalArgumentException();
    }

    /**
     * {@inheritDoc}
     *
     * @see java.util.NavigableSet#headSet(Object, boolean)
     * @since 1.6
     */
    @SuppressWarnings("unchecked")
    public NavigableSet<E> headSet(E end, boolean endInclusive) {
        // Check for errors
        Comparator<? super E> c = backingMap.comparator();
        if (c == null) {
            ((Comparable<E>) end).compareTo(end);
        } else {
            c.compare(end, end);
        }
        return new TreeSet<E>(backingMap.headMap(end, endInclusive));
    }

    /**
     * {@inheritDoc}
     *
     * @see java.util.NavigableSet#tailSet(Object, boolean)
     * @since 1.6
     */
    @SuppressWarnings("unchecked")
    public NavigableSet<E> tailSet(E start, boolean startInclusive) {
        // Check for errors
        Comparator<? super E> c = backingMap.comparator();
        if (c == null) {
            ((Comparable<E>) start).compareTo(start);
        } else {
            c.compare(start, start);
        }
        return new TreeSet<E>(backingMap.tailMap(start, startInclusive));
    }

    /**
     * Returns a {@code SortedSet} of the specified portion of this {@code TreeSet} which
     * contains elements greater or equal to the start element but less than the
     * end element. The returned SortedSet is backed by this TreeSet so changes
     * to one are reflected by the other.
     *
     * @param start
     *            the start element
     * @param end
     *            the end element
     * @return a subset where the elements are greater or equal to
     *         <code>start</code> and less than <code>end</code>
     *
     * @exception ClassCastException
     *                when the start or end object cannot be compared with the
     *                elements in this TreeSet
     * @exception NullPointerException
     *                when the start or end object is null and the comparator
     *                cannot handle null
     */
    @SuppressWarnings("unchecked")
    public SortedSet<E> subSet(E start, E end) {
        return subSet(start, true, end, false);
    }

    /**
     * Returns a {@code SortedSet} of the specified portion of this {@code TreeSet} which
     * contains elements less than the end element. The returned SortedSet is
     * backed by this TreeSet so changes to one are reflected by the other.
     *
     * @param end
     *            the end element
     * @return a subset where the elements are less than <code>end</code>
     *
     * @exception ClassCastException
     *                when the end object cannot be compared with the elements
     *                in this TreeSet
     * @exception NullPointerException
     *                when the end object is null and the comparator cannot
     *                handle null
     */
    @SuppressWarnings("unchecked")
    public SortedSet<E> headSet(E end) {
        return headSet(end, false);
    }

    /**
     * Returns a {@code SortedSet} of the specified portion of this {@code TreeSet} which
     * contains elements greater or equal to the start element. The returned
     * SortedSet is backed by this TreeSet so changes to one are reflected by
     * the other.
     *
     * @param start
     *            the start element
     * @return a subset where the elements are greater or equal to
     *         <code>start</code>
     *
     * @exception ClassCastException
     *                when the start object cannot be compared with the elements
     *                in this TreeSet
     * @exception NullPointerException
     *                when the start object is null and the comparator cannot
     *                handle null
     */
    @SuppressWarnings("unchecked")
    public SortedSet<E> tailSet(E start) {
        return tailSet(start, true);
    }

    private void writeObject(ObjectOutputStream stream) throws IOException {
        stream.defaultWriteObject();
        stream.writeObject(backingMap.comparator());
        int size = backingMap.size();
        stream.writeInt(size);
        if (size > 0) {
            Iterator<E> it = backingMap.keySet().iterator();
            while (it.hasNext()) {
                stream.writeObject(it.next());
            }
        }
    }

    @SuppressWarnings("unchecked")
    private void readObject(ObjectInputStream stream) throws IOException,
            ClassNotFoundException {
        stream.defaultReadObject();
        TreeMap<E, Object> map = new TreeMap<E, Object>(
                (Comparator<? super E>) stream.readObject());
        int size = stream.readInt();
        if (size > 0) {
            for (int i=0; i<size; i++) {
                E elem = (E)stream.readObject();
                map.put(elem, Boolean.TRUE);
            }
        }
        backingMap = map;
    }
}

AbstractList AbstractClass

/*
 *  Licensed to the Apache Software Foundation (ASF) under one or more
 *  contributor license agreements.  See the NOTICE file distributed with
 *  this work for additional information regarding copyright ownership.
 *  The ASF licenses this file to You under the Apache License, Version 2.0
 *  (the "License"); you may not use this file except in compliance with
 *  the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 *  Unless required by applicable law or agreed to in writing, software
 *  distributed under the License is distributed on an "AS IS" BASIS,
 *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *  See the License for the specific language governing permissions and
 *  limitations under the License.
 */

package java.util;


public abstract class AbstractList<E> extends AbstractCollection<E> implements List<E> {

    /**
     * A counter for changes to the list.
     */
    protected transient int modCount;

    private class SimpleListIterator implements Iterator<E> {
        int pos = -1;

        int expectedModCount;

        int lastPosition = -1;

        SimpleListIterator() {
            expectedModCount = modCount;
        }

        public boolean hasNext() {
            return pos + 1 < size();
        }

        public E next() {
            if (expectedModCount == modCount) {
                try {
                    E result = get(pos + 1);
                    lastPosition = ++pos;
                    return result;
                } catch (IndexOutOfBoundsException e) {
                    throw new NoSuchElementException();
                }
            }
            throw new ConcurrentModificationException();
        }

        public void remove() {
            if (this.lastPosition == -1) {
                throw new IllegalStateException();
            }

            if (expectedModCount != modCount) {
                throw new ConcurrentModificationException();
            }

            try {
                AbstractList.this.remove(lastPosition);
            } catch (IndexOutOfBoundsException e) {
                throw new ConcurrentModificationException();
            }

            expectedModCount = modCount;
            if (pos == lastPosition) {
                pos--;
            }
            lastPosition = -1;
        }
    }

    private final class FullListIterator extends SimpleListIterator implements ListIterator<E> {
        FullListIterator(int start) {
            if (start >= 0 && start <= size()) {
                pos = start - 1;
            } else {
                throw new IndexOutOfBoundsException();
            }
        }

        public void add(E object) {
            if (expectedModCount == modCount) {
                try {
                    AbstractList.this.add(pos + 1, object);
                } catch (IndexOutOfBoundsException e) {
                    throw new NoSuchElementException();
                }
                pos++;
                lastPosition = -1;
                if (modCount != expectedModCount) {
                    expectedModCount = modCount;
                }
            } else {
                throw new ConcurrentModificationException();
            }
        }

        public boolean hasPrevious() {
            return pos >= 0;
        }

        public int nextIndex() {
            return pos + 1;
        }

        public E previous() {
            if (expectedModCount == modCount) {
                try {
                    E result = get(pos);
                    lastPosition = pos;
                    pos--;
                    return result;
                } catch (IndexOutOfBoundsException e) {
                    throw new NoSuchElementException();
                }
            }
            throw new ConcurrentModificationException();
        }

        public int previousIndex() {
            return pos;
        }

        public void set(E object) {
            if (expectedModCount == modCount) {
                try {
                    AbstractList.this.set(lastPosition, object);
                } catch (IndexOutOfBoundsException e) {
                    throw new IllegalStateException();
                }
            } else {
                throw new ConcurrentModificationException();
            }
        }
    }

    private static final class SubAbstractListRandomAccess<E> extends
            SubAbstractList<E> implements RandomAccess {
        SubAbstractListRandomAccess(AbstractList<E> list, int start, int end) {
            super(list, start, end);
        }
    }

    private static class SubAbstractList<E> extends AbstractList<E> {
        private final AbstractList<E> fullList;

        private int offset;

        private int size;

        private static final class SubAbstractListIterator<E> implements
                ListIterator<E> {
            private final SubAbstractList<E> subList;

            private final ListIterator<E> iterator;

            private int start;

            private int end;

            SubAbstractListIterator(ListIterator<E> it,
                    SubAbstractList<E> list, int offset, int length) {
                iterator = it;
                subList = list;
                start = offset;
                end = start + length;
            }

            public void add(E object) {
                iterator.add(object);
                subList.sizeChanged(true);
                end++;
            }

            public boolean hasNext() {
                return iterator.nextIndex() < end;
            }

            public boolean hasPrevious() {
                return iterator.previousIndex() >= start;
            }

            public E next() {
                if (iterator.nextIndex() < end) {
                    return iterator.next();
                }
                throw new NoSuchElementException();
            }

            public int nextIndex() {
                return iterator.nextIndex() - start;
            }

            public E previous() {
                if (iterator.previousIndex() >= start) {
                    return iterator.previous();
                }
                throw new NoSuchElementException();
            }

            public int previousIndex() {
                int previous = iterator.previousIndex();
                if (previous >= start) {
                    return previous - start;
                }
                return -1;
            }

            public void remove() {
                iterator.remove();
                subList.sizeChanged(false);
                end--;
            }

            public void set(E object) {
                iterator.set(object);
            }
        }

        SubAbstractList(AbstractList<E> list, int start, int end) {
            fullList = list;
            modCount = fullList.modCount;
            offset = start;
            size = end - start;
        }

        @Override
        public void add(int location, E object) {
            if (modCount == fullList.modCount) {
                if (location >= 0 && location <= size) {
                    fullList.add(location + offset, object);
                    size++;
                    modCount = fullList.modCount;
                } else {
                    throw new IndexOutOfBoundsException();
                }
            } else {
                throw new ConcurrentModificationException();
            }
        }

        @Override
        public boolean addAll(int location, Collection<? extends E> collection) {
            if (modCount == fullList.modCount) {
                if (location >= 0 && location <= size) {
                    boolean result = fullList.addAll(location + offset,
                            collection);
                    if (result) {
                        size += collection.size();
                        modCount = fullList.modCount;
                    }
                    return result;
                }
                throw new IndexOutOfBoundsException();
            }
            throw new ConcurrentModificationException();
        }

        @Override
        public boolean addAll(Collection<? extends E> collection) {
            if (modCount == fullList.modCount) {
                boolean result = fullList.addAll(offset + size, collection);
                if (result) {
                    size += collection.size();
                    modCount = fullList.modCount;
                }
                return result;
            }
            throw new ConcurrentModificationException();
        }

        @Override
        public E get(int location) {
            if (modCount == fullList.modCount) {
                if (location >= 0 && location < size) {
                    return fullList.get(location + offset);
                }
                throw new IndexOutOfBoundsException();
            }
            throw new ConcurrentModificationException();
        }

        @Override
        public Iterator<E> iterator() {
            return listIterator(0);
        }

        @Override
        public ListIterator<E> listIterator(int location) {
            if (modCount == fullList.modCount) {
                if (location >= 0 && location <= size) {
                    return new SubAbstractListIterator<E>(fullList
                            .listIterator(location + offset), this, offset,
                            size);
                }
                throw new IndexOutOfBoundsException();
            }
            throw new ConcurrentModificationException();
        }

        @Override
        public E remove(int location) {
            if (modCount == fullList.modCount) {
                if (location >= 0 && location < size) {
                    E result = fullList.remove(location + offset);
                    size--;
                    modCount = fullList.modCount;
                    return result;
                }
                throw new IndexOutOfBoundsException();
            }
            throw new ConcurrentModificationException();
        }

        @Override
        protected void removeRange(int start, int end) {
            if (start != end) {
                if (modCount == fullList.modCount) {
                    fullList.removeRange(start + offset, end + offset);
                    size -= end - start;
                    modCount = fullList.modCount;
                } else {
                    throw new ConcurrentModificationException();
                }
            }
        }

        @Override
        public E set(int location, E object) {
            if (modCount == fullList.modCount) {
                if (location >= 0 && location < size) {
                    return fullList.set(location + offset, object);
                }
                throw new IndexOutOfBoundsException();
            }
            throw new ConcurrentModificationException();
        }

        @Override
        public int size() {
            if (modCount == fullList.modCount) {
                return size;
            }
            throw new ConcurrentModificationException();
        }

        void sizeChanged(boolean increment) {
            if (increment) {
                size++;
            } else {
                size--;
            }
            modCount = fullList.modCount;
        }
    }

    /**
     * Constructs a new instance of this AbstractList.
     */
    protected AbstractList() {
    }

    /**
     * Inserts the specified object into this List at the specified location.
     * The object is inserted before any previous element at the specified
     * location. If the location is equal to the size of this List, the object
     * is added at the end.
     * <p>
     * Concrete implementations that would like to support the add functionality
     * must override this method.
     *
     * @param location
     *            the index at which to insert.
     * @param object
     *            the object to add.
     *
     * @throws UnsupportedOperationException
     *                if adding to this List is not supported.
     * @throws ClassCastException
     *                if the class of the object is inappropriate for this
     *                List
     * @throws IllegalArgumentException
     *                if the object cannot be added to this List
     * @throws IndexOutOfBoundsException
     *                if {@code location < 0 || >= size()}
     */
    public void add(int location, E object) {
        throw new UnsupportedOperationException();
    }

    /**
     * Adds the specified object at the end of this List.
     *
     *
     * @param object
     *            the object to add
     * @return true
     *
     * @throws UnsupportedOperationException
     *                if adding to this List is not supported
     * @throws ClassCastException
     *                if the class of the object is inappropriate for this
     *                List
     * @throws IllegalArgumentException
     *                if the object cannot be added to this List
     */
    @Override
    public boolean add(E object) {
        add(size(), object);
        return true;
    }

    /**
     * Inserts the objects in the specified Collection at the specified location
     * in this List. The objects are added in the order they are returned from
     * the collection's iterator.
     *
     * @param location
     *            the index at which to insert.
     * @param collection
     *            the Collection of objects
     * @return {@code true} if this List is modified, {@code false} otherwise.
     * @throws UnsupportedOperationException
     *             if adding to this list is not supported.
     * @throws ClassCastException
     *             if the class of an object is inappropriate for this list.
     * @throws IllegalArgumentException
     *             if an object cannot be added to this list.
     * @throws IndexOutOfBoundsException
     *             if {@code location < 0 || > size()}
     */
    public boolean addAll(int location, Collection<? extends E> collection) {
        Iterator<? extends E> it = collection.iterator();
        while (it.hasNext()) {
            add(location++, it.next());
        }
        return !collection.isEmpty();
    }

    /**
     * Removes all elements from this list, leaving it empty.
     *
     * @throws UnsupportedOperationException
     *             if removing from this list is not supported.
     * @see List#isEmpty
     * @see List#size
     */
    @Override
    public void clear() {
        removeRange(0, size());
    }

    @Override
    public boolean equals(Object object) {
        if (this == object) {
            return true;
        }
        if (object instanceof List) {
            List<?> list = (List<?>) object;
            if (list.size() != size()) {
                return false;
            }

            Iterator<?> it1 = iterator(), it2 = list.iterator();
            while (it1.hasNext()) {
                Object e1 = it1.next(), e2 = it2.next();
                if (!(e1 == null ? e2 == null : e1.equals(e2))) {
                    return false;
                }
            }
            return true;
        }
        return false;
    }
    public abstract E get(int location);

    @Override
    public int hashCode() {
        int result = 1;
        Iterator<?> it = iterator();
        while (it.hasNext()) {
            Object object = it.next();
            result = (31 * result) + (object == null ? 0 : object.hashCode());
        }
        return result;
    }
    public int indexOf(Object object) {
        ListIterator<?> it = listIterator();
        if (object != null) {
            while (it.hasNext()) {
                if (object.equals(it.next())) {
                    return it.previousIndex();
                }
            }
        } else {
            while (it.hasNext()) {
                if (it.next() == null) {
                    return it.previousIndex();
                }
            }
        }
        return -1;
    }

    @Override
    public Iterator<E> iterator() {
        return new SimpleListIterator();
    }

    public int lastIndexOf(Object object) {
        ListIterator<?> it = listIterator(size());
        if (object != null) {
            while (it.hasPrevious()) {
                if (object.equals(it.previous())) {
                    return it.nextIndex();
                }
            }
        } else {
            while (it.hasPrevious()) {
                if (it.previous() == null) {
                    return it.nextIndex();
                }
            }
        }
        return -1;
    }
 
    public ListIterator<E> listIterator() {
        return listIterator(0);
    }

    public ListIterator<E> listIterator(int location) {
        return new FullListIterator(location);
    }


    public E remove(int location) {
        throw new UnsupportedOperationException();
    }

    protected void removeRange(int start, int end) {
        Iterator<?> it = listIterator(start);
        for (int i = start; i < end; i++) {
            it.next();
            it.remove();
        }
    }

    public E set(int location, E object) {
        throw new UnsupportedOperationException();
    }

    public List<E> subList(int start, int end) {
        if (start >= 0 && end <= size()) {
            if (start <= end) {
                if (this instanceof RandomAccess) {
                    return new SubAbstractListRandomAccess<E>(this, start, end);
                }
                return new SubAbstractList<E>(this, start, end);
            }
            throw new IllegalArgumentException();
        }
        throw new IndexOutOfBoundsException();
    }
}

Vector

/*
 *  Licensed to the Apache Software Foundation (ASF) under one or more
 *  contributor license agreements.  See the NOTICE file distributed with
 *  this work for additional information regarding copyright ownership.
 *  The ASF licenses this file to You under the Apache License, Version 2.0
 *  (the "License"); you may not use this file except in compliance with
 *  the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 *  Unless required by applicable law or agreed to in writing, software
 *  distributed under the License is distributed on an "AS IS" BASIS,
 *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *  See the License for the specific language governing permissions and
 *  limitations under the License.
 */

package java.util;

import java.io.IOException;
import java.io.ObjectOutputStream;
import java.io.Serializable;
import java.lang.reflect.Array;

/**
 * Vector is an implementation of {@link List}, backed by an array and synchronized.
 * All optional operations including adding, removing, and replacing elements are supported.
 *
 * <p>All elements are permitted, including null.
 *
 * <p>This class is equivalent to {@link ArrayList} with synchronized operations. This has a
 * performance cost, and the synchronization is not necessarily meaningful to your application:
 * synchronizing each call to {@code get}, for example, is not equivalent to synchronizing on the
 * list and iterating over it (which is probably what you intended). If you do need very highly
 * concurrent access, you should also consider {@link java.util.concurrent.CopyOnWriteArrayList}.
 *
 * @param <E> The element type of this list.
 */
public class Vector<E> extends AbstractList<E> implements List<E>,
        RandomAccess, Cloneable, Serializable {

    private static final long serialVersionUID = -2767605614048989439L;

    /**
     * The number of elements or the size of the vector.
     */
    protected int elementCount;

    /**
     * The elements of the vector.
     */
    protected Object[] elementData;

    /**
     * How many elements should be added to the vector when it is detected that
     * it needs to grow to accommodate extra entries. If this value is zero or
     * negative the size will be doubled if an increase is needed.
     */
    protected int capacityIncrement;

    private static final int DEFAULT_SIZE = 10;

    /**
     * Constructs a new vector using the default capacity.
     */
    public Vector() {
        this(DEFAULT_SIZE, 0);
    }

    /**
     * Constructs a new vector using the specified capacity.
     *
     * @param capacity
     *            the initial capacity of the new vector.
     * @throws IllegalArgumentException
     *             if {@code capacity} is negative.
     */
    public Vector(int capacity) {
        this(capacity, 0);
    }

    /**
     * Constructs a new vector using the specified capacity and capacity
     * increment.
     *
     * @param capacity
     *            the initial capacity of the new vector.
     * @param capacityIncrement
     *            the amount to increase the capacity when this vector is full.
     * @throws IllegalArgumentException
     *             if {@code capacity} is negative.
     */
    public Vector(int capacity, int capacityIncrement) {
        if (capacity < 0) {
            throw new IllegalArgumentException();
        }
        elementData = newElementArray(capacity);
        elementCount = 0;
        this.capacityIncrement = capacityIncrement;
    }

    /**
     * Constructs a new instance of {@code Vector} containing the elements in
     * {@code collection}. The order of the elements in the new {@code Vector}
     * is dependent on the iteration order of the seed collection.
     *
     * @param collection
     *            the collection of elements to add.
     */
    public Vector(Collection<? extends E> collection) {
        this(collection.size(), 0);
        Iterator<? extends E> it = collection.iterator();
        while (it.hasNext()) {
            elementData[elementCount++] = it.next();
        }
    }

    @SuppressWarnings("unchecked")
    private E[] newElementArray(int size) {
        return (E[]) new Object[size];
    }

    /**
     * Adds the specified object into this vector at the specified location. The
     * object is inserted before any element with the same or a higher index
     * increasing their index by 1. If the location is equal to the size of this
     * vector, the object is added at the end.
     *
     * @param location
     *            the index at which to insert the element.
     * @param object
     *            the object to insert in this vector.
     * @throws ArrayIndexOutOfBoundsException
     *                if {@code location < 0 || location > size()}.
     * @see #addElement
     * @see #size
     */
    @Override
    public void add(int location, E object) {
        insertElementAt(object, location);
    }

    /**
     * Adds the specified object at the end of this vector.
     *
     * @param object
     *            the object to add to the vector.
     * @return {@code true}
     */
    @Override
    public synchronized boolean add(E object) {
        if (elementCount == elementData.length) {
            growByOne();
        }
        elementData[elementCount++] = object;
        modCount++;
        return true;
    }

    /**
     * Inserts the objects in the specified collection at the specified location
     * in this vector. The objects are inserted in the order in which they are
     * returned from the Collection iterator. The elements with an index equal
     * or higher than {@code location} have their index increased by the size of
     * the added collection.
     *
     * @param location
     *            the location to insert the objects.
     * @param collection
     *            the collection of objects.
     * @return {@code true} if this vector is modified, {@code false} otherwise.
     * @throws ArrayIndexOutOfBoundsException
     *                if {@code location < 0} or {@code location > size()}.
     */
    @Override
    public synchronized boolean addAll(int location, Collection<? extends E> collection) {
        if (location >= 0 && location <= elementCount) {
            int size = collection.size();
            if (size == 0) {
                return false;
            }
            int required = size - (elementData.length - elementCount);
            if (required > 0) {
                growBy(required);
            }
            int count = elementCount - location;
            if (count > 0) {
                System.arraycopy(elementData, location, elementData, location
                        + size, count);
            }
            Iterator<? extends E> it = collection.iterator();
            while (it.hasNext()) {
                elementData[location++] = it.next();
            }
            elementCount += size;
            modCount++;
            return true;
        }
        throw arrayIndexOutOfBoundsException(location, elementCount);
    }

    /**
     * Adds the objects in the specified collection to the end of this vector.
     *
     * @param collection
     *            the collection of objects.
     * @return {@code true} if this vector is modified, {@code false} otherwise.
     */
    @Override
    public synchronized boolean addAll(Collection<? extends E> collection) {
        return addAll(elementCount, collection);
    }

    /**
     * Adds the specified object at the end of this vector.
     *
     * @param object
     *            the object to add to the vector.
     */
    public synchronized void addElement(E object) {
        if (elementCount == elementData.length) {
            growByOne();
        }
        elementData[elementCount++] = object;
        modCount++;
    }

    /**
     * Returns the number of elements this vector can hold without growing.
     *
     * @return the capacity of this vector.
     * @see #ensureCapacity
     * @see #size
     */
    public synchronized int capacity() {
        return elementData.length;
    }

    /**
     * Removes all elements from this vector, leaving it empty.
     *
     * @see #isEmpty
     * @see #size
     */
    @Override
    public void clear() {
        removeAllElements();
    }

    /**
     * Returns a new vector with the same elements, size, capacity and capacity
     * increment as this vector.
     *
     * @return a shallow copy of this vector.
     * @see java.lang.Cloneable
     */
    @Override
    @SuppressWarnings("unchecked")
    public synchronized Object clone() {
        try {
            Vector<E> vector = (Vector<E>) super.clone();
            vector.elementData = elementData.clone();
            return vector;
        } catch (CloneNotSupportedException e) {
            throw new AssertionError(e);
        }
    }

    /**
     * Searches this vector for the specified object.
     *
     * @param object
     *            the object to look for in this vector.
     * @return {@code true} if object is an element of this vector,
     *         {@code false} otherwise.
     * @see #indexOf(Object)
     * @see #indexOf(Object, int)
     * @see java.lang.Object#equals
     */
    @Override
    public boolean contains(Object object) {
        return indexOf(object, 0) != -1;
    }

    /**
     * Searches this vector for all objects in the specified collection.
     *
     * @param collection
     *            the collection of objects.
     * @return {@code true} if all objects in the specified collection are
     *         elements of this vector, {@code false} otherwise.
     */
    @Override
    public synchronized boolean containsAll(Collection<?> collection) {
        return super.containsAll(collection);
    }

    /**
     * Attempts to copy elements contained by this {@code Vector} into the
     * corresponding elements of the supplied {@code Object} array.
     *
     * @param elements
     *            the {@code Object} array into which the elements of this
     *            vector are copied.
     * @throws IndexOutOfBoundsException
     *             if {@code elements} is not big enough.
     * @see #clone
     */
    public synchronized void copyInto(Object[] elements) {
        System.arraycopy(elementData, 0, elements, 0, elementCount);
    }

    /**
     * Returns the element at the specified location in this vector.
     *
     * @param location
     *            the index of the element to return in this vector.
     * @return the element at the specified location.
     * @throws ArrayIndexOutOfBoundsException
     *                if {@code location < 0 || location >= size()}.
     * @see #size
     */
    @SuppressWarnings("unchecked")
    public synchronized E elementAt(int location) {
        if (location < elementCount) {
            return (E) elementData[location];
        }
        throw arrayIndexOutOfBoundsException(location, elementCount);
    }

    /**
     * Returns an enumeration on the elements of this vector. The results of the
     * enumeration may be affected if the contents of this vector is modified.
     *
     * @return an enumeration of the elements of this vector.
     * @see #elementAt
     * @see Enumeration
     */
    public Enumeration<E> elements() {
        return new Enumeration<E>() {
            int pos = 0;

            public boolean hasMoreElements() {
                return pos < elementCount;
            }

            @SuppressWarnings("unchecked")
            public E nextElement() {
                synchronized (Vector.this) {
                    if (pos < elementCount) {
                        return (E) elementData[pos++];
                    }
                }
                throw new NoSuchElementException();
            }
        };
    }

    /**
     * Ensures that this vector can hold the specified number of elements
     * without growing.
     *
     * @param minimumCapacity
     *            the minimum number of elements that this vector will hold
     *            before growing.
     * @see #capacity
     */
    public synchronized void ensureCapacity(int minimumCapacity) {
        if (elementData.length < minimumCapacity) {
            int next = (capacityIncrement <= 0 ? elementData.length
                    : capacityIncrement)
                    + elementData.length;
            grow(minimumCapacity > next ? minimumCapacity : next);
        }
    }

    /**
     * Compares the specified object to this vector and returns if they are
     * equal. The object must be a List which contains the same objects in the
     * same order.
     *
     * @param object
     *            the object to compare with this object
     * @return {@code true} if the specified object is equal to this vector,
     *         {@code false} otherwise.
     * @see #hashCode
     */
    @Override
    public synchronized boolean equals(Object object) {
        if (this == object) {
            return true;
        }
        if (object instanceof List) {
            List<?> list = (List<?>) object;
            if (list.size() != elementCount) {
                return false;
            }

            int index = 0;
            Iterator<?> it = list.iterator();
            while (it.hasNext()) {
                Object e1 = elementData[index++], e2 = it.next();
                if (!(e1 == null ? e2 == null : e1.equals(e2))) {
                    return false;
                }
            }
            return true;
        }
        return false;
    }

    /**
     * Returns the first element in this vector.
     *
     * @return the element at the first position.
     * @throws NoSuchElementException
     *                if this vector is empty.
     * @see #elementAt
     * @see #lastElement
     * @see #size
     */
    @SuppressWarnings("unchecked")
    public synchronized E firstElement() {
        if (elementCount > 0) {
            return (E) elementData[0];
        }
        throw new NoSuchElementException();
    }

    /**
     * Returns the element at the specified location in this vector.
     *
     * @param location
     *            the index of the element to return in this vector.
     * @return the element at the specified location.
     * @throws ArrayIndexOutOfBoundsException
     *                if {@code location < 0 || location >= size()}.
     * @see #size
     */
    @Override
    public E get(int location) {
        return elementAt(location);
    }

    private void grow(int newCapacity) {
        E[] newData = newElementArray(newCapacity);
        // Assumes elementCount is <= newCapacity
        assert elementCount <= newCapacity;
        System.arraycopy(elementData, 0, newData, 0, elementCount);
        elementData = newData;
    }

    /**
     * JIT optimization
     */
    private void growByOne() {
        int adding = 0;
        if (capacityIncrement <= 0) {
            if ((adding = elementData.length) == 0) {
                adding = 1;
            }
        } else {
            adding = capacityIncrement;
        }

        E[] newData = newElementArray(elementData.length + adding);
        System.arraycopy(elementData, 0, newData, 0, elementCount);
        elementData = newData;
    }

    private void growBy(int required) {
        int adding = 0;
        if (capacityIncrement <= 0) {
            if ((adding = elementData.length) == 0) {
                adding = required;
            }
            while (adding < required) {
                adding += adding;
            }
        } else {
            adding = (required / capacityIncrement) * capacityIncrement;
            if (adding < required) {
                adding += capacityIncrement;
            }
        }
        E[] newData = newElementArray(elementData.length + adding);
        System.arraycopy(elementData, 0, newData, 0, elementCount);
        elementData = newData;
    }

    /**
     * Returns an integer hash code for the receiver. Objects which are equal
     * return the same value for this method.
     *
     * @return the receiver's hash.
     * @see #equals
     */
    @Override
    public synchronized int hashCode() {
        int result = 1;
        for (int i = 0; i < elementCount; i++) {
            result = (31 * result)
                    + (elementData[i] == null ? 0 : elementData[i].hashCode());
        }
        return result;
    }

    /**
     * Searches in this vector for the index of the specified object. The search
     * for the object starts at the beginning and moves towards the end of this
     * vector.
     *
     * @param object
     *            the object to find in this vector.
     * @return the index in this vector of the specified element, -1 if the
     *         element isn't found.
     * @see #contains
     * @see #lastIndexOf(Object)
     * @see #lastIndexOf(Object, int)
     */
    @Override
    public int indexOf(Object object) {
        return indexOf(object, 0);
    }

    /**
     * Searches in this vector for the index of the specified object. The search
     * for the object starts at the specified location and moves towards the end
     * of this vector.
     *
     * @param object
     *            the object to find in this vector.
     * @param location
     *            the index at which to start searching.
     * @return the index in this vector of the specified element, -1 if the
     *         element isn't found.
     * @throws ArrayIndexOutOfBoundsException
     *                if {@code location < 0}.
     * @see #contains
     * @see #lastIndexOf(Object)
     * @see #lastIndexOf(Object, int)
     */
    public synchronized int indexOf(Object object, int location) {
        if (object != null) {
            for (int i = location; i < elementCount; i++) {
                if (object.equals(elementData[i])) {
                    return i;
                }
            }
        } else {
            for (int i = location; i < elementCount; i++) {
                if (elementData[i] == null) {
                    return i;
                }
            }
        }
        return -1;
    }

    /**
     * Inserts the specified object into this vector at the specified location.
     * This object is inserted before any previous element at the specified
     * location. All elements with an index equal or greater than
     * {@code location} have their index increased by 1. If the location is
     * equal to the size of this vector, the object is added at the end.
     *
     * @param object
     *            the object to insert in this vector.
     * @param location
     *            the index at which to insert the element.
     * @throws ArrayIndexOutOfBoundsException
     *                if {@code location < 0 || location > size()}.
     * @see #addElement
     * @see #size
     */
    public synchronized void insertElementAt(E object, int location) {
        if (location >= 0 && location <= elementCount) {
            if (elementCount == elementData.length) {
                growByOne();
            }
            int count = elementCount - location;
            if (count > 0) {
                System.arraycopy(elementData, location, elementData,
                        location + 1, count);
            }
            elementData[location] = object;
            elementCount++;
            modCount++;
        } else {
            throw arrayIndexOutOfBoundsException(location, elementCount);
        }
    }

    /**
     * Returns if this vector has no elements, a size of zero.
     *
     * @return {@code true} if this vector has no elements, {@code false}
     *         otherwise.
     * @see #size
     */
    @Override
    public synchronized boolean isEmpty() {
        return elementCount == 0;
    }

    /**
     * Returns the last element in this vector.
     *
     * @return the element at the last position.
     * @throws NoSuchElementException
     *                if this vector is empty.
     * @see #elementAt
     * @see #firstElement
     * @see #size
     */
    @SuppressWarnings("unchecked")
    public synchronized E lastElement() {
        try {
            return (E) elementData[elementCount - 1];
        } catch (IndexOutOfBoundsException e) {
            throw new NoSuchElementException();
        }
    }

    /**
     * Searches in this vector for the index of the specified object. The search
     * for the object starts at the end and moves towards the start of this
     * vector.
     *
     * @param object
     *            the object to find in this vector.
     * @return the index in this vector of the specified element, -1 if the
     *         element isn't found.
     * @see #contains
     * @see #indexOf(Object)
     * @see #indexOf(Object, int)
     */
    @Override
    public synchronized int lastIndexOf(Object object) {
        return lastIndexOf(object, elementCount - 1);
    }

    /**
     * Searches in this vector for the index of the specified object. The search
     * for the object starts at the specified location and moves towards the
     * start of this vector.
     *
     * @param object
     *            the object to find in this vector.
     * @param location
     *            the index at which to start searching.
     * @return the index in this vector of the specified element, -1 if the
     *         element isn't found.
     * @throws ArrayIndexOutOfBoundsException
     *                if {@code location >= size()}.
     * @see #contains
     * @see #indexOf(Object)
     * @see #indexOf(Object, int)
     */
    public synchronized int lastIndexOf(Object object, int location) {
        if (location < elementCount) {
            if (object != null) {
                for (int i = location; i >= 0; i--) {
                    if (object.equals(elementData[i])) {
                        return i;
                    }
                }
            } else {
                for (int i = location; i >= 0; i--) {
                    if (elementData[i] == null) {
                        return i;
                    }
                }
            }
            return -1;
        }
        throw arrayIndexOutOfBoundsException(location, elementCount);
    }

    /**
     * Removes the object at the specified location from this vector. All
     * elements with an index bigger than {@code location} have their index
     * decreased by 1.
     *
     * @param location
     *            the index of the object to remove.
     * @return the removed object.
     * @throws IndexOutOfBoundsException
     *                if {@code location < 0 || location >= size()}.
     */
    @SuppressWarnings("unchecked")
    @Override
    public synchronized E remove(int location) {
        if (location < elementCount) {
            E result = (E) elementData[location];
            elementCount--;
            int size = elementCount - location;
            if (size > 0) {
                System.arraycopy(elementData, location + 1, elementData,
                        location, size);
            }
            elementData[elementCount] = null;
            modCount++;
            return result;
        }
        throw arrayIndexOutOfBoundsException(location, elementCount);
    }

    /**
     * Removes the first occurrence, starting at the beginning and moving
     * towards the end, of the specified object from this vector. All elements
     * with an index bigger than the element that gets removed have their index
     * decreased by 1.
     *
     * @param object
     *            the object to remove from this vector.
     * @return {@code true} if the specified object was found, {@code false}
     *         otherwise.
     * @see #removeAllElements
     * @see #removeElementAt
     * @see #size
     */
    @Override
    public boolean remove(Object object) {
        return removeElement(object);
    }

    /**
     * Removes all occurrences in this vector of each object in the specified
     * Collection.
     *
     * @param collection
     *            the collection of objects to remove.
     * @return {@code true} if this vector is modified, {@code false} otherwise.
     * @see #remove(Object)
     * @see #contains(Object)
     */
    @Override
    public synchronized boolean removeAll(Collection<?> collection) {
        return super.removeAll(collection);
    }

    /**
     * Removes all elements from this vector, leaving the size zero and the
     * capacity unchanged.
     *
     * @see #isEmpty
     * @see #size
     */
    public synchronized void removeAllElements() {
        for (int i = 0; i < elementCount; i++) {
            elementData[i] = null;
        }
        modCount++;
        elementCount = 0;
    }

    /**
     * Removes the first occurrence, starting at the beginning and moving
     * towards the end, of the specified object from this vector. All elements
     * with an index bigger than the element that gets removed have their index
     * decreased by 1.
     *
     * @param object
     *            the object to remove from this vector.
     * @return {@code true} if the specified object was found, {@code false}
     *         otherwise.
     * @see #removeAllElements
     * @see #removeElementAt
     * @see #size
     */
    public synchronized boolean removeElement(Object object) {
        int index;
        if ((index = indexOf(object, 0)) == -1) {
            return false;
        }
        removeElementAt(index);
        return true;
    }

    /**
     * Removes the element found at index position {@code location} from
     * this {@code Vector}. All elements with an index bigger than
     * {@code location} have their index decreased by 1.
     *
     * @param location
     *            the index of the element to remove.
     * @throws ArrayIndexOutOfBoundsException
     *                if {@code location < 0 || location >= size()}.
     * @see #removeElement
     * @see #removeAllElements
     * @see #size
     */
    public synchronized void removeElementAt(int location) {
        if (location >= 0 && location < elementCount) {
            elementCount--;
            int size = elementCount - location;
            if (size > 0) {
                System.arraycopy(elementData, location + 1, elementData,
                        location, size);
            }
            elementData[elementCount] = null;
            modCount++;
        } else {
            throw arrayIndexOutOfBoundsException(location, elementCount);
        }
    }

    /**
     * Removes the objects in the specified range from the start to the, but not
     * including, end index. All elements with an index bigger than or equal to
     * {@code end} have their index decreased by {@code end - start}.
     *
     * @param start
     *            the index at which to start removing.
     * @param end
     *            the index one past the end of the range to remove.
     * @throws IndexOutOfBoundsException
     *                if {@code start < 0, start > end} or
     *                {@code end > size()}.
     */
    @Override
    protected void removeRange(int start, int end) {
        if (start >= 0 && start <= end && end <= elementCount) {
            if (start == end) {
                return;
            }
            if (end != elementCount) {
                System.arraycopy(elementData, end, elementData, start,
                        elementCount - end);
                int newCount = elementCount - (end - start);
                Arrays.fill(elementData, newCount, elementCount, null);
                elementCount = newCount;
            } else {
                Arrays.fill(elementData, start, elementCount, null);
                elementCount = start;
            }
            modCount++;
        } else {
            throw new IndexOutOfBoundsException();
        }
    }

    /**
     * Removes all objects from this vector that are not contained in the
     * specified collection.
     *
     * @param collection
     *            the collection of objects to retain.
     * @return {@code true} if this vector is modified, {@code false} otherwise.
     * @see #remove(Object)
     */
    @Override
    public synchronized boolean retainAll(Collection<?> collection) {
        return super.retainAll(collection);
    }

    /**
     * Replaces the element at the specified location in this vector with the
     * specified object.
     *
     * @param location
     *            the index at which to put the specified object.
     * @param object
     *            the object to add to this vector.
     * @return the previous element at the location.
     * @throws ArrayIndexOutOfBoundsException
     *                if {@code location < 0 || location >= size()}.
     * @see #size
     */
    @SuppressWarnings("unchecked")
    @Override
    public synchronized E set(int location, E object) {
        if (location < elementCount) {
            E result = (E) elementData[location];
            elementData[location] = object;
            return result;
        }
        throw arrayIndexOutOfBoundsException(location, elementCount);
    }

    /**
     * Replaces the element at the specified location in this vector with the
     * specified object.
     *
     * @param object
     *            the object to add to this vector.
     * @param location
     *            the index at which to put the specified object.
     * @throws ArrayIndexOutOfBoundsException
     *                if {@code location < 0 || location >= size()}.
     * @see #size
     */
    public synchronized void setElementAt(E object, int location) {
        if (location < elementCount) {
            elementData[location] = object;
        } else {
            throw arrayIndexOutOfBoundsException(location, elementCount);
        }
    }

    /**
     * This method was extracted to encourage VM to inline callers.
     * TODO: when we have a VM that can actually inline, move the test in here too!
     */
    private static ArrayIndexOutOfBoundsException arrayIndexOutOfBoundsException(int index, int size) {
        throw new ArrayIndexOutOfBoundsException(size, index);
    }

    /**
     * Sets the size of this vector to the specified size. If there are more
     * than length elements in this vector, the elements at end are lost. If
     * there are less than length elements in the vector, the additional
     * elements contain null.
     *
     * @param length
     *            the new size of this vector.
     * @see #size
     */
    public synchronized void setSize(int length) {
        if (length == elementCount) {
            return;
        }
        ensureCapacity(length);
        if (elementCount > length) {
            Arrays.fill(elementData, length, elementCount, null);
        }
        elementCount = length;
        modCount++;
    }

    /**
     * Returns the number of elements in this vector.
     *
     * @return the number of elements in this vector.
     * @see #elementCount
     * @see #lastElement
     */
    @Override
    public synchronized int size() {
        return elementCount;
    }

    /**
     * Returns a List of the specified portion of this vector from the start
     * index to one less than the end index. The returned List is backed by this
     * vector so changes to one are reflected by the other.
     *
     * @param start
     *            the index at which to start the sublist.
     * @param end
     *            the index one past the end of the sublist.
     * @return a List of a portion of this vector.
     * @throws IndexOutOfBoundsException
     *                if {@code start < 0} or {@code end > size()}.
     * @throws IllegalArgumentException
     *                if {@code start > end}.
     */
    @Override
    public synchronized List<E> subList(int start, int end) {
        return new Collections.SynchronizedRandomAccessList<E>(super.subList(
                start, end), this);
    }

    /**
     * Returns a new array containing all elements contained in this vector.
     *
     * @return an array of the elements from this vector.
     */
    @Override
    public synchronized Object[] toArray() {
        Object[] result = new Object[elementCount];
        System.arraycopy(elementData, 0, result, 0, elementCount);
        return result;
    }

    /**
     * Returns an array containing all elements contained in this vector. If the
     * specified array is large enough to hold the elements, the specified array
     * is used, otherwise an array of the same type is created. If the specified
     * array is used and is larger than this vector, the array element following
     * the collection elements is set to null.
     *
     * @param contents
     *            the array to fill.
     * @return an array of the elements from this vector.
     * @throws ArrayStoreException
     *                if the type of an element in this vector cannot be
     *                stored in the type of the specified array.
     */
    @Override
    @SuppressWarnings("unchecked")
    public synchronized <T> T[] toArray(T[] contents) {
        if (elementCount > contents.length) {
            Class<?> ct = contents.getClass().getComponentType();
            contents = (T[]) Array.newInstance(ct, elementCount);
        }
        System.arraycopy(elementData, 0, contents, 0, elementCount);
        if (elementCount < contents.length) {
            contents[elementCount] = null;
        }
        return contents;
    }

    /**
     * Returns the string representation of this vector.
     *
     * @return the string representation of this vector.
     * @see #elements
     */
    @Override
    public synchronized String toString() {
        if (elementCount == 0) {
            return "[]";
        }
        int length = elementCount - 1;
        StringBuilder buffer = new StringBuilder(elementCount * 16);
        buffer.append('[');
        for (int i = 0; i < length; i++) {
            if (elementData[i] == this) {
                buffer.append("(this Collection)");
            } else {
                buffer.append(elementData[i]);
            }
            buffer.append(", ");
        }
        if (elementData[length] == this) {
            buffer.append("(this Collection)");
        } else {
            buffer.append(elementData[length]);
        }
        buffer.append(']');
        return buffer.toString();
    }

    /**
     * Sets the capacity of this vector to be the same as the size.
     *
     * @see #capacity
     * @see #ensureCapacity
     * @see #size
     */
    public synchronized void trimToSize() {
        if (elementData.length != elementCount) {
            grow(elementCount);
        }
    }

    private synchronized void writeObject(ObjectOutputStream stream)
            throws IOException {
        stream.defaultWriteObject();
    }
}

ArrayList

/*
 *  Licensed to the Apache Software Foundation (ASF) under one or more
 *  contributor license agreements.  See the NOTICE file distributed with
 *  this work for additional information regarding copyright ownership.
 *  The ASF licenses this file to You under the Apache License, Version 2.0
 *  (the "License"); you may not use this file except in compliance with
 *  the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 *  Unless required by applicable law or agreed to in writing, software
 *  distributed under the License is distributed on an "AS IS" BASIS,
 *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *  See the License for the specific language governing permissions and
 *  limitations under the License.
 */

package java.util;

import java.io.IOException;
import java.io.InvalidObjectException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;
import java.lang.reflect.Array;
import libcore.util.EmptyArray;

/**
 * ArrayList is an implementation of {@link List}, backed by an array.
 * All optional operations including adding, removing, and replacing elements are supported.
 *
 * <p>All elements are permitted, including null.
 *
 * <p>This class is a good choice as your default {@code List} implementation.
 * {@link Vector} synchronizes all operations, but not necessarily in a way that's
 * meaningful to your application: synchronizing each call to {@code get}, for example, is not
 * equivalent to synchronizing the list and iterating over it (which is probably what you intended).
 * {@link java.util.concurrent.CopyOnWriteArrayList} is intended for the special case of very high
 * concurrency, frequent traversals, and very rare mutations.
 *
 * @param <E> The element type of this list.
 * @since 1.2
 */
public class ArrayList<E> extends AbstractList<E> implements Cloneable, Serializable, RandomAccess {
    /**
     * The minimum amount by which the capacity of an ArrayList will increase.
     * This tuning parameter controls a time-space tradeoff. This value (12)
     * gives empirically good results and is arguably consistent with the
     * RI's specified default initial capacity of 10: instead of 10, we start
     * with 0 (sans allocation) and jump to 12.
     */
    private static final int MIN_CAPACITY_INCREMENT = 12;

    /**
     * The number of elements in this list.
     */
    int size;

    /**
     * The elements in this list, followed by nulls.
     */
    transient Object[] array;

    /**
     * Constructs a new instance of {@code ArrayList} with the specified
     * initial capacity.
     *
     * @param capacity
     *            the initial capacity of this {@code ArrayList}.
     */
    public ArrayList(int capacity) {
        if (capacity < 0) {
            throw new IllegalArgumentException();
        }
        array = (capacity == 0 ? EmptyArray.OBJECT : new Object[capacity]);
    }

    /**
     * Constructs a new {@code ArrayList} instance with zero initial capacity.
     */
    public ArrayList() {
        array = EmptyArray.OBJECT;
    }

    /**
     * Constructs a new instance of {@code ArrayList} containing the elements of
     * the specified collection.
     *
     * @param collection
     *            the collection of elements to add.
     */
    public ArrayList(Collection<? extends E> collection) {
        Object[] a = collection.toArray();
        if (a.getClass() != Object[].class) {
            Object[] newArray = new Object[a.length];
            System.arraycopy(a, 0, newArray, 0, a.length);
            a = newArray;
        }
        array = a;
        size = a.length;
    }

    /**
     * Adds the specified object at the end of this {@code ArrayList}.
     *
     * @param object
     *            the object to add.
     * @return always true
     */
    @Override public boolean add(E object) {
        Object[] a = array;
        int s = size;
        if (s == a.length) {
            Object[] newArray = new Object[s +
                    (s < (MIN_CAPACITY_INCREMENT / 2) ?
                     MIN_CAPACITY_INCREMENT : s >> 1)];
            System.arraycopy(a, 0, newArray, 0, s);
            array = a = newArray;
        }
        a[s] = object;
        size = s + 1;
        modCount++;
        return true;
    }

    /**
     * Inserts the specified object into this {@code ArrayList} at the specified
     * location. The object is inserted before any previous element at the
     * specified location. If the location is equal to the size of this
     * {@code ArrayList}, the object is added at the end.
     *
     * @param index
     *            the index at which to insert the object.
     * @param object
     *            the object to add.
     * @throws IndexOutOfBoundsException
     *             when {@code location < 0 || > size()}
     */
    @Override public void add(int index, E object) {
        Object[] a = array;
        int s = size;
        if (index > s || index < 0) {
            throwIndexOutOfBoundsException(index, s);
        }

        if (s < a.length) {
            System.arraycopy(a, index, a, index + 1, s - index);
        } else {
            // assert s == a.length;
            Object[] newArray = new Object[newCapacity(s)];
            System.arraycopy(a, 0, newArray, 0, index);
            System.arraycopy(a, index, newArray, index + 1, s - index);
            array = a = newArray;
        }
        a[index] = object;
        size = s + 1;
        modCount++;
    }

    /**
     * This method controls the growth of ArrayList capacities.  It represents
     * a time-space tradeoff: we don't want to grow lists too frequently
     * (which wastes time and fragments storage), but we don't want to waste
     * too much space in unused excess capacity.
     *
     * NOTE: This method is inlined into {@link #add(Object)} for performance.
     * If you change the method, change it there too!
     */
    private static int newCapacity(int currentCapacity) {
        int increment = (currentCapacity < (MIN_CAPACITY_INCREMENT / 2) ?
                MIN_CAPACITY_INCREMENT : currentCapacity >> 1);
        return currentCapacity + increment;
    }

    /**
     * Adds the objects in the specified collection to this {@code ArrayList}.
     *
     * @param collection
     *            the collection of objects.
     * @return {@code true} if this {@code ArrayList} is modified, {@code false}
     *         otherwise.
     */
    @Override public boolean addAll(Collection<? extends E> collection) {
        Object[] newPart = collection.toArray();
        int newPartSize = newPart.length;
        if (newPartSize == 0) {
            return false;
        }
        Object[] a = array;
        int s = size;
        int newSize = s + newPartSize; // If add overflows, arraycopy will fail
        if (newSize > a.length) {
            int newCapacity = newCapacity(newSize - 1);  // ~33% growth room
            Object[] newArray = new Object[newCapacity];
            System.arraycopy(a, 0, newArray, 0, s);
            array = a = newArray;
        }
        System.arraycopy(newPart, 0, a, s, newPartSize);
        size = newSize;
        modCount++;
        return true;
    }

    /**
     * Inserts the objects in the specified collection at the specified location
     * in this List. The objects are added in the order they are returned from
     * the collection's iterator.
     *
     * @param index
     *            the index at which to insert.
     * @param collection
     *            the collection of objects.
     * @return {@code true} if this {@code ArrayList} is modified, {@code false}
     *         otherwise.
     * @throws IndexOutOfBoundsException
     *             when {@code location < 0 || > size()}
     */
    @Override
    public boolean addAll(int index, Collection<? extends E> collection) {
        int s = size;
        if (index > s || index < 0) {
            throwIndexOutOfBoundsException(index, s);
        }
        Object[] newPart = collection.toArray();
        int newPartSize = newPart.length;
        if (newPartSize == 0) {
            return false;
        }
        Object[] a = array;
        int newSize = s + newPartSize; // If add overflows, arraycopy will fail
        if (newSize <= a.length) {
             System.arraycopy(a, index, a, index + newPartSize, s - index);
        } else {
            int newCapacity = newCapacity(newSize - 1);  // ~33% growth room
            Object[] newArray = new Object[newCapacity];
            System.arraycopy(a, 0, newArray, 0, index);
            System.arraycopy(a, index, newArray, index + newPartSize, s-index);
            array = a = newArray;
        }
        System.arraycopy(newPart, 0, a, index, newPartSize);
        size = newSize;
        modCount++;
        return true;
    }

    /**
     * This method was extracted to encourage VM to inline callers.
     * TODO: when we have a VM that can actually inline, move the test in here too!
     */
    static IndexOutOfBoundsException throwIndexOutOfBoundsException(int index, int size) {
        throw new IndexOutOfBoundsException("Invalid index " + index + ", size is " + size);
    }

    /**
     * Removes all elements from this {@code ArrayList}, leaving it empty.
     *
     * @see #isEmpty
     * @see #size
     */
    @Override public void clear() {
        if (size != 0) {
            Arrays.fill(array, 0, size, null);
            size = 0;
            modCount++;
        }
    }

    /**
     * Returns a new {@code ArrayList} with the same elements, the same size and
     * the same capacity as this {@code ArrayList}.
     *
     * @return a shallow copy of this {@code ArrayList}
     * @see java.lang.Cloneable
     */
    @Override public Object clone() {
        try {
            ArrayList<?> result = (ArrayList<?>) super.clone();
            result.array = array.clone();
            return result;
        } catch (CloneNotSupportedException e) {
           throw new AssertionError();
        }
    }

    /**
     * Ensures that after this operation the {@code ArrayList} can hold the
     * specified number of elements without further growing.
     *
     * @param minimumCapacity
     *            the minimum capacity asked for.
     */
    public void ensureCapacity(int minimumCapacity) {
        Object[] a = array;
        if (a.length < minimumCapacity) {
            Object[] newArray = new Object[minimumCapacity];
            System.arraycopy(a, 0, newArray, 0, size);
            array = newArray;
            modCount++;
        }
    }

    @SuppressWarnings("unchecked") @Override public E get(int index) {
        if (index >= size) {
            throwIndexOutOfBoundsException(index, size);
        }
        return (E) array[index];
    }

    /**
     * Returns the number of elements in this {@code ArrayList}.
     *
     * @return the number of elements in this {@code ArrayList}.
     */
    @Override public int size() {
        return size;
    }

    @Override public boolean isEmpty() {
        return size == 0;
    }

    /**
     * Searches this {@code ArrayList} for the specified object.
     *
     * @param object
     *            the object to search for.
     * @return {@code true} if {@code object} is an element of this
     *         {@code ArrayList}, {@code false} otherwise
     */
    @Override public boolean contains(Object object) {
        Object[] a = array;
        int s = size;
        if (object != null) {
            for (int i = 0; i < s; i++) {
                if (object.equals(a[i])) {
                    return true;
                }
            }
        } else {
            for (int i = 0; i < s; i++) {
                if (a[i] == null) {
                    return true;
                }
            }
        }
        return false;
    }

    @Override public int indexOf(Object object) {
        Object[] a = array;
        int s = size;
        if (object != null) {
            for (int i = 0; i < s; i++) {
                if (object.equals(a[i])) {
                    return i;
                }
            }
        } else {
            for (int i = 0; i < s; i++) {
                if (a[i] == null) {
                    return i;
                }
            }
        }
        return -1;
    }

    @Override public int lastIndexOf(Object object) {
        Object[] a = array;
        if (object != null) {
            for (int i = size - 1; i >= 0; i--) {
                if (object.equals(a[i])) {
                    return i;
                }
            }
        } else {
            for (int i = size - 1; i >= 0; i--) {
                if (a[i] == null) {
                    return i;
                }
            }
        }
        return -1;
    }

    /**
     * Removes the object at the specified location from this list.
     *
     * @param index
     *            the index of the object to remove.
     * @return the removed object.
     * @throws IndexOutOfBoundsException
     *             when {@code location < 0 || >= size()}
     */
    @Override public E remove(int index) {
        Object[] a = array;
        int s = size;
        if (index >= s) {
            throwIndexOutOfBoundsException(index, s);
        }
        @SuppressWarnings("unchecked") E result = (E) a[index];
        System.arraycopy(a, index + 1, a, index, --s - index);
        a[s] = null;  // Prevent memory leak
        size = s;
        modCount++;
        return result;
    }

    @Override public boolean remove(Object object) {
        Object[] a = array;
        int s = size;
        if (object != null) {
            for (int i = 0; i < s; i++) {
                if (object.equals(a[i])) {
                    System.arraycopy(a, i + 1, a, i, --s - i);
                    a[s] = null;  // Prevent memory leak
                    size = s;
                    modCount++;
                    return true;
                }
            }
        } else {
            for (int i = 0; i < s; i++) {
                if (a[i] == null) {
                    System.arraycopy(a, i + 1, a, i, --s - i);
                    a[s] = null;  // Prevent memory leak
                    size = s;
                    modCount++;
                    return true;
                }
            }
        }
        return false;
    }

    @Override protected void removeRange(int fromIndex, int toIndex) {
        if (fromIndex == toIndex) {
            return;
        }
        Object[] a = array;
        int s = size;
        if (fromIndex >= s) {
            throw new IndexOutOfBoundsException("fromIndex " + fromIndex
                    + " >= size " + size);
        }
        if (toIndex > s) {
            throw new IndexOutOfBoundsException("toIndex " + toIndex
                    + " > size " + size);
        }
        if (fromIndex > toIndex) {
            throw new IndexOutOfBoundsException("fromIndex " + fromIndex
                    + " > toIndex " + toIndex);
        }

        System.arraycopy(a, toIndex, a, fromIndex, s - toIndex);
        int rangeSize = toIndex - fromIndex;
        Arrays.fill(a, s - rangeSize, s, null);
        size = s - rangeSize;
        modCount++;
    }

    /**
     * Replaces the element at the specified location in this {@code ArrayList}
     * with the specified object.
     *
     * @param index
     *            the index at which to put the specified object.
     * @param object
     *            the object to add.
     * @return the previous element at the index.
     * @throws IndexOutOfBoundsException
     *             when {@code location < 0 || >= size()}
     */
    @Override public E set(int index, E object) {
        Object[] a = array;
        if (index >= size) {
            throwIndexOutOfBoundsException(index, size);
        }
        @SuppressWarnings("unchecked") E result = (E) a[index];
        a[index] = object;
        return result;
    }

    /**
     * Returns a new array containing all elements contained in this
     * {@code ArrayList}.
     *
     * @return an array of the elements from this {@code ArrayList}
     */
    @Override public Object[] toArray() {
        int s = size;
        Object[] result = new Object[s];
        System.arraycopy(array, 0, result, 0, s);
        return result;
    }

    /**
     * Returns an array containing all elements contained in this
     * {@code ArrayList}. If the specified array is large enough to hold the
     * elements, the specified array is used, otherwise an array of the same
     * type is created. If the specified array is used and is larger than this
     * {@code ArrayList}, the array element following the collection elements
     * is set to null.
     *
     * @param contents
     *            the array.
     * @return an array of the elements from this {@code ArrayList}.
     * @throws ArrayStoreException
     *             when the type of an element in this {@code ArrayList} cannot
     *             be stored in the type of the specified array.
     */
    @Override public <T> T[] toArray(T[] contents) {
        int s = size;
        if (contents.length < s) {
            @SuppressWarnings("unchecked") T[] newArray
                = (T[]) Array.newInstance(contents.getClass().getComponentType(), s);
            contents = newArray;
        }
        System.arraycopy(this.array, 0, contents, 0, s);
        if (contents.length > s) {
            contents[s] = null;
        }
        return contents;
    }

    /**
     * Sets the capacity of this {@code ArrayList} to be the same as the current
     * size.
     *
     * @see #size
     */
    public void trimToSize() {
        int s = size;
        if (s == array.length) {
            return;
        }
        if (s == 0) {
            array = EmptyArray.OBJECT;
        } else {
            Object[] newArray = new Object[s];
            System.arraycopy(array, 0, newArray, 0, s);
            array = newArray;
        }
        modCount++;
    }

    @Override public Iterator<E> iterator() {
        return new ArrayListIterator();
    }

    private class ArrayListIterator implements Iterator<E> {
        /** Number of elements remaining in this iteration */
        private int remaining = size;

        /** Index of element that remove() would remove, or -1 if no such elt */
        private int removalIndex = -1;

        /** The expected modCount value */
        private int expectedModCount = modCount;

        public boolean hasNext() {
            return remaining != 0;
        }

        @SuppressWarnings("unchecked") public E next() {
            ArrayList<E> ourList = ArrayList.this;
            int rem = remaining;
            if (ourList.modCount != expectedModCount) {
                throw new ConcurrentModificationException();
            }
            if (rem == 0) {
                throw new NoSuchElementException();
            }
            remaining = rem - 1;
            return (E) ourList.array[removalIndex = ourList.size - rem];
        }

        public void remove() {
            Object[] a = array;
            int removalIdx = removalIndex;
            if (modCount != expectedModCount) {
                throw new ConcurrentModificationException();
            }
            if (removalIdx < 0) {
                throw new IllegalStateException();
            }
            System.arraycopy(a, removalIdx + 1, a, removalIdx, remaining);
            a[--size] = null;  // Prevent memory leak
            removalIndex = -1;
            expectedModCount = ++modCount;
        }
    }

    @Override public int hashCode() {
        Object[] a = array;
        int hashCode = 1;
        for (int i = 0, s = size; i < s; i++) {
            Object e = a[i];
            hashCode = 31 * hashCode + (e == null ? 0 : e.hashCode());
        }
        return hashCode;
    }

    @Override public boolean equals(Object o) {
        if (o == this) {
            return true;
        }
        if (!(o instanceof List)) {
            return false;
        }
        List<?> that = (List<?>) o;
        int s = size;
        if (that.size() != s) {
            return false;
        }
        Object[] a = array;
        if (that instanceof RandomAccess) {
            for (int i = 0; i < s; i++) {
                Object eThis = a[i];
                Object ethat = that.get(i);
                if (eThis == null ? ethat != null : !eThis.equals(ethat)) {
                    return false;
                }
            }
        } else {  // Argument list is not random access; use its iterator
            Iterator<?> it = that.iterator();
            for (int i = 0; i < s; i++) {
                Object eThis = a[i];
                Object eThat = it.next();
                if (eThis == null ? eThat != null : !eThis.equals(eThat)) {
                    return false;
                }
            }
        }
        return true;
    }

    private static final long serialVersionUID = 8683452581122892189L;

    private void writeObject(ObjectOutputStream stream) throws IOException {
        stream.defaultWriteObject();
        stream.writeInt(array.length);
        for (int i = 0; i < size; i++) {
            stream.writeObject(array[i]);
        }
    }

    private void readObject(ObjectInputStream stream) throws IOException, ClassNotFoundException {
        stream.defaultReadObject();
        int cap = stream.readInt();
        if (cap < size) {
            throw new InvalidObjectException(
                    "Capacity: " + cap + " < size: " + size);
        }
        array = (cap == 0 ? EmptyArray.OBJECT : new Object[cap]);
        for (int i = 0; i < size; i++) {
            array[i] = stream.readObject();
        }
    }
 }

package java.util;
public abstract class Dictionary<K, V> {   
    public Dictionary() {
    }
    public abstract Enumeration<V> elements();
    public abstract V get(Object key);
    public abstract boolean isEmpty();
    public abstract Enumeration<K> keys();
    public abstract V put(K key, V value);
    public abstract V remove(Object key);
    public abstract int size();
}

Map Interface

package java.util;

public interface Map<K,V> {

    /**
     * {@code Map.Entry} is a key/value mapping contained in a {@code Map}.
     */
    public static interface Entry<K,V> {     
        public boolean equals(Object object);
        public K getKey();
        public V getValue();
        public int hashCode();
        public V setValue(V object);
    };
    public void clear();    
    public boolean containsKey(Object key);
    public boolean containsValue(Object value);
    public Set<Map.Entry<K,V>> entrySet();
    public boolean equals(Object object);
    public V get(Object key);
    public int hashCode();
    public boolean isEmpty();
    public Set<K> keySet();
    public V put(K key, V value);
    public void putAll(Map<? extends K,? extends V> map);
    public V remove(Object key);
    public int size();
    public Collection<V> values();
}

SortedMap Interface

package java.util;

public interface SortedMap<K,V> extends Map<K,V> {
    
    public Comparator<? super K> comparator();
    public K firstKey();
    public SortedMap<K,V> headMap(K endKey);
    public K lastKey();
    public SortedMap<K,V> subMap(K startKey, K endKey);
    public SortedMap<K,V> tailMap(K startKey);
}

AbstractMap

package java.util;
import java.io.Serializable;
public abstract class AbstractMap<K, V> implements Map<K, V> {

    // Lazily initialized key set.
    Set<K> keySet;
    Collection<V> valuesCollection;
    public static class SimpleImmutableEntry<K, V>
            implements Map.Entry<K, V>, Serializable {
        private static final long serialVersionUID = 7138329143949025153L;

        private final K key;
        private final V value;

        public SimpleImmutableEntry(K theKey, V theValue) {
            key = theKey;
            value = theValue;
        }
        public SimpleImmutableEntry(Map.Entry<? extends K, ? extends V> copyFrom) {
            key = copyFrom.getKey();
            value = copyFrom.getValue();
        }
        public K getKey() {
            return key;
        }
        public V getValue() {
            return value;
        }
        public V setValue(V object) {
            throw new UnsupportedOperationException();
        }
        @Override 
        public boolean equals(Object object) {
            if (this == object) {
                return true;
            }
            if (object instanceof Map.Entry) {
                Map.Entry<?, ?> entry = (Map.Entry<?, ?>) object;
                return (key == null ? entry.getKey() == null : key.equals(entry
                        .getKey()))
                        && (value == null ? entry.getValue() == null : value
                                .equals(entry.getValue()));
            }
            return false;
        }
        @Override public int hashCode() {
            return (key == null ? 0 : key.hashCode())
                    ^ (value == null ? 0 : value.hashCode());
        }
        @Override public String toString() {
            return key + "=" + value;
        }
    }
    public static class SimpleEntry<K, V> implements Map.Entry<K, V>, Serializable {
        private static final long serialVersionUID = -8499721149061103585L;

        private final K key;
        private V value;

        public SimpleEntry(K theKey, V theValue) {
            key = theKey;
            value = theValue;
        }
        public SimpleEntry(Map.Entry<? extends K, ? extends V> copyFrom) {
            key = copyFrom.getKey();
            value = copyFrom.getValue();
        }
        public K getKey() {
            return key;
        }
        public V getValue() {
            return value;
        }
        public V setValue(V object) {
            V result = value;
            value = object;
            return result;
        }
        @Override 
        public boolean equals(Object object) {
            if (this == object) {
                return true;
            }
            if (object instanceof Map.Entry) {
                Map.Entry<?, ?> entry = (Map.Entry<?, ?>) object;
                return (key == null ? entry.getKey() == null : key.equals(entry
                        .getKey()))
                        && (value == null ? entry.getValue() == null : value
                                .equals(entry.getValue()));
            }
            return false;
        }
        @Override 
        public int hashCode() {
            return (key == null ? 0 : key.hashCode())
                    ^ (value == null ? 0 : value.hashCode());
        }
        @Override 
        public String toString() {
            return key + "=" + value;
        }
    }
    protected AbstractMap() {
    }

    public void clear() {
        entrySet().clear();
    }

    public boolean containsKey(Object key) {
        Iterator<Map.Entry<K, V>> it = entrySet().iterator();
        if (key != null) {
            while (it.hasNext()) {
                if (key.equals(it.next().getKey())) {
                    return true;
                }
            }
        } else {
            while (it.hasNext()) {
                if (it.next().getKey() == null) {
                    return true;
                }
            }
        }
        return false;
    }
    public boolean containsValue(Object value) {
        Iterator<Map.Entry<K, V>> it = entrySet().iterator();
        if (value != null) {
            while (it.hasNext()) {
                if (value.equals(it.next().getValue())) {
                    return true;
                }
            }
        } else {
            while (it.hasNext()) {
                if (it.next().getValue() == null) {
                    return true;
                }
            }
        }
        return false;
    }

    public abstract Set<Map.Entry<K, V>> entrySet();
    @Override 
    public boolean equals(Object object) {
        if (this == object) {
            return true;
        }
        if (object instanceof Map) {
            Map<?, ?> map = (Map<?, ?>) object;
            if (size() != map.size()) {
                return false;
            }

            try {
                for (Entry<K, V> entry : entrySet()) {
                    K key = entry.getKey();
                    V mine = entry.getValue();
                    Object theirs = map.get(key);
                    if (mine == null) {
                        if (theirs != null || !map.containsKey(key)) {
                            return false;
                        }
                    } else if (!mine.equals(theirs)) {
                        return false;
                    }
                }
            } catch (NullPointerException ignored) {
                return false;
            } catch (ClassCastException ignored) {
                return false;
            }
            return true;
        }
        return false;
    }
    public V get(Object key) {
        Iterator<Map.Entry<K, V>> it = entrySet().iterator();
        if (key != null) {
            while (it.hasNext()) {
                Map.Entry<K, V> entry = it.next();
                if (key.equals(entry.getKey())) {
                    return entry.getValue();
                }
            }
        } else {
            while (it.hasNext()) {
                Map.Entry<K, V> entry = it.next();
                if (entry.getKey() == null) {
                    return entry.getValue();
                }
            }
        }
        return null;
    }
    @Override public int hashCode() {
        int result = 0;
        Iterator<Map.Entry<K, V>> it = entrySet().iterator();
        while (it.hasNext()) {
            result += it.next().hashCode();
        }
        return result;
    }
    public boolean isEmpty() {
        return size() == 0;
    }
    public Set<K> keySet() {
        if (keySet == null) {
            keySet = new AbstractSet<K>() {
                @Override public boolean contains(Object object) {
                    return containsKey(object);
                }

                @Override public int size() {
                    return AbstractMap.this.size();
                }

                @Override public Iterator<K> iterator() {
                    return new Iterator<K>() {
                        Iterator<Map.Entry<K, V>> setIterator = entrySet().iterator();

                        public boolean hasNext() {
                            return setIterator.hasNext();
                        }

                        public K next() {
                            return setIterator.next().getKey();
                        }

                        public void remove() {
                            setIterator.remove();
                        }
                    };
                }
            };
        }
        return keySet;
    }
    public V put(K key, V value) {
        throw new UnsupportedOperationException();
    }
    public void putAll(Map<? extends K, ? extends V> map) {
        for (Map.Entry<? extends K, ? extends V> entry : map.entrySet()) {
            put(entry.getKey(), entry.getValue());
        }
    }
    public V remove(Object key) {
        Iterator<Map.Entry<K, V>> it = entrySet().iterator();
        if (key != null) {
            while (it.hasNext()) {
                Map.Entry<K, V> entry = it.next();
                if (key.equals(entry.getKey())) {
                    it.remove();
                    return entry.getValue();
                }
            }
        } else {
            while (it.hasNext()) {
                Map.Entry<K, V> entry = it.next();
                if (entry.getKey() == null) {
                    it.remove();
                    return entry.getValue();
                }
            }
        }
        return null;
    }
    public int size() {
        return entrySet().size();
    }
    @Override public String toString() {
        if (isEmpty()) {
            return "{}";
        }

        StringBuilder buffer = new StringBuilder(size() * 28);
        buffer.append('{');
        Iterator<Map.Entry<K, V>> it = entrySet().iterator();
        while (it.hasNext()) {
            Map.Entry<K, V> entry = it.next();
            Object key = entry.getKey();
            if (key != this) {
                buffer.append(key);
            } else {
                buffer.append("(this Map)");
            }
            buffer.append('=');
            Object value = entry.getValue();
            if (value != this) {
                buffer.append(value);
            } else {
                buffer.append("(this Map)");
            }
            if (it.hasNext()) {
                buffer.append(", ");
            }
        }
        buffer.append('}');
        return buffer.toString();
    }
    public Collection<V> values() {
        if (valuesCollection == null) {
            valuesCollection = new AbstractCollection<V>() {
                @Override public int size() {
                    return AbstractMap.this.size();
                }

                @Override public boolean contains(Object object) {
                    return containsValue(object);
                }

                @Override public Iterator<V> iterator() {
                    return new Iterator<V>() {
                        Iterator<Map.Entry<K, V>> setIterator = entrySet().iterator();

                        public boolean hasNext() {
                            return setIterator.hasNext();
                        }

                        public V next() {
                            return setIterator.next().getValue();
                        }

                        public void remove() {
                            setIterator.remove();
                        }
                    };
                }
            };
        }
        return valuesCollection;
    }
    @SuppressWarnings("unchecked")
    @Override 
    protected Object clone() throws CloneNotSupportedException {
        AbstractMap<K, V> result = (AbstractMap<K, V>) super.clone();
        result.keySet = null;
        result.valuesCollection = null;
        return result;
    }
}

 HashMap

package java.util;
import java.io.IOException;
import java.io.InvalidObjectException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.ObjectStreamField;
import java.io.Serializable;
import java.util.Map.Entry;

import libcore.util.Objects;

/**
 * HashMap is an implementation of {@link Map}. All optional operations are supported.
 *
 * <p>All elements are permitted as keys or values, including null.
 *
 * <p>Note that the iteration order for HashMap is non-deterministic. If you want
 * deterministic iteration, use {@link LinkedHashMap}.
 *
 * <p>Note: the implementation of {@code HashMap} is not synchronized.
 * If one thread of several threads accessing an instance modifies the map
 * structurally, access to the map needs to be synchronized. A structural
 * modification is an operation that adds or removes an entry. Changes in
 * the value of an entry are not structural changes.
 *
 * <p>The {@code Iterator} created by calling the {@code iterator} method
 * may throw a {@code ConcurrentModificationException} if the map is structurally
 * changed while an iterator is used to iterate over the elements. Only the
 * {@code remove} method that is provided by the iterator allows for removal of
 * elements during iteration. It is not possible to guarantee that this
 * mechanism works in all cases of unsynchronized concurrent modification. It
 * should only be used for debugging purposes.
 *
 * @param <K> the type of keys maintained by this map
 * @param <V> the type of mapped values
 */
public class HashMap<K, V> extends AbstractMap<K, V> implements Cloneable, Serializable {
    /**
     * Min capacity (other than zero) for a HashMap. Must be a power of two
     * greater than 1 (and less than 1 << 30).
     */
    private static final int MINIMUM_CAPACITY = 4;

    /**
     * Max capacity for a HashMap. Must be a power of two >= MINIMUM_CAPACITY.
     */
    private static final int MAXIMUM_CAPACITY = 1 << 30;

    /**
     * An empty table shared by all zero-capacity maps (typically from default
     * constructor). It is never written to, and replaced on first put. Its size
     * is set to half the minimum, so that the first resize will create a
     * minimum-sized table.
     */
    private static final Entry[] EMPTY_TABLE
            = new HashMapEntry[MINIMUM_CAPACITY >>> 1];

    /**
     * The default load factor. Note that this implementation ignores the
     * load factor, but cannot do away with it entirely because it's
     * mentioned in the API.
     *
     * <p>Note that this constant has no impact on the behavior of the program,
     * but it is emitted as part of the serialized form. The load factor of
     * .75 is hardwired into the program, which uses cheap shifts in place of
     * expensive division.
     */
    static final float DEFAULT_LOAD_FACTOR = .75F;

    /**
     * The hash table. If this hash map contains a mapping for null, it is
     * not represented this hash table.
     */
    transient HashMapEntry<K, V>[] table;

    /**
     * The entry representing the null key, or null if there's no such mapping.
     */
    transient HashMapEntry<K, V> entryForNullKey;

    /**
     * The number of mappings in this hash map.
     */
    transient int size;

    /**
     * Incremented by "structural modifications" to allow (best effort)
     * detection of concurrent modification.
     */
    transient int modCount;

    /**
     * The table is rehashed when its size exceeds this threshold.
     * The value of this field is generally .75 * capacity, except when
     * the capacity is zero, as described in the EMPTY_TABLE declaration
     * above.
     */
    private transient int threshold;

    // Views - lazily initialized
    private transient Set<K> keySet;
    private transient Set<Entry<K, V>> entrySet;
    private transient Collection<V> values;

    /**
     * Constructs a new empty {@code HashMap} instance.
     */
    @SuppressWarnings("unchecked")
    public HashMap() {
        table = (HashMapEntry<K, V>[]) EMPTY_TABLE;
        threshold = -1; // Forces first put invocation to replace EMPTY_TABLE
    }

    /**
     * Constructs a new {@code HashMap} instance with the specified capacity.
     *
     * @param capacity
     *            the initial capacity of this hash map.
     * @throws IllegalArgumentException
     *                when the capacity is less than zero.
     */
    public HashMap(int capacity) {
        if (capacity < 0) {
            throw new IllegalArgumentException("Capacity: " + capacity);
        }

        if (capacity == 0) {
            @SuppressWarnings("unchecked")
            HashMapEntry<K, V>[] tab = (HashMapEntry<K, V>[]) EMPTY_TABLE;
            table = tab;
            threshold = -1; // Forces first put() to replace EMPTY_TABLE
            return;
        }

        if (capacity < MINIMUM_CAPACITY) {
            capacity = MINIMUM_CAPACITY;
        } else if (capacity > MAXIMUM_CAPACITY) {
            capacity = MAXIMUM_CAPACITY;
        } else {
            capacity = roundUpToPowerOfTwo(capacity);
        }
        makeTable(capacity);
    }

    /**
     * Constructs a new {@code HashMap} instance with the specified capacity and
     * load factor.
     *
     * @param capacity
     *            the initial capacity of this hash map.
     * @param loadFactor
     *            the initial load factor.
     * @throws IllegalArgumentException
     *                when the capacity is less than zero or the load factor is
     *                less or equal to zero or NaN.
     */
    public HashMap(int capacity, float loadFactor) {
        this(capacity);

        if (loadFactor <= 0 || Float.isNaN(loadFactor)) {
            throw new IllegalArgumentException("Load factor: " + loadFactor);
        }

        /*
         * Note that this implementation ignores loadFactor; it always uses
         * a load factor of 3/4. This simplifies the code and generally
         * improves performance.
         */
    }

    /**
     * Constructs a new {@code HashMap} instance containing the mappings from
     * the specified map.
     *
     * @param map
     *            the mappings to add.
     */
    public HashMap(Map<? extends K, ? extends V> map) {
        this(capacityForInitSize(map.size()));
        constructorPutAll(map);
    }

    /**
     * Inserts all of the elements of map into this HashMap in a manner
     * suitable for use by constructors and pseudo-constructors (i.e., clone,
     * readObject). Also used by LinkedHashMap.
     */
    final void constructorPutAll(Map<? extends K, ? extends V> map) {
        for (Entry<? extends K, ? extends V> e : map.entrySet()) {
            constructorPut(e.getKey(), e.getValue());
        }
    }

    /**
     * Returns an appropriate capacity for the specified initial size. Does
     * not round the result up to a power of two; the caller must do this!
     * The returned value will be between 0 and MAXIMUM_CAPACITY (inclusive).
     */
    static int capacityForInitSize(int size) {
        int result = (size >> 1) + size; // Multiply by 3/2 to allow for growth

        // boolean expr is equivalent to result >= 0 && result<MAXIMUM_CAPACITY
        return (result & ~(MAXIMUM_CAPACITY-1))==0 ? result : MAXIMUM_CAPACITY;
    }

    /**
     * Returns a shallow copy of this map.
     *
     * @return a shallow copy of this map.
     */
    @SuppressWarnings("unchecked")
    @Override public Object clone() {
        /*
         * This could be made more efficient. It unnecessarily hashes all of
         * the elements in the map.
         */
        HashMap<K, V> result;
        try {
            result = (HashMap<K, V>) super.clone();
        } catch (CloneNotSupportedException e) {
            throw new AssertionError(e);
        }

        // Restore clone to empty state, retaining our capacity and threshold
        result.makeTable(table.length);
        result.entryForNullKey = null;
        result.size = 0;
        result.keySet = null;
        result.entrySet = null;
        result.values = null;

        result.init(); // Give subclass a chance to initialize itself
        result.constructorPutAll(this); // Calls method overridden in subclass!!
        return result;
    }

    /**
     * This method is called from the pseudo-constructors (clone and readObject)
     * prior to invoking constructorPut/constructorPutAll, which invoke the
     * overridden constructorNewEntry method. Normally it is a VERY bad idea to
     * invoke an overridden method from a pseudo-constructor (Effective Java
     * Item 17). In this case it is unavoidable, and the init method provides a
     * workaround.
     */
    void init() { }

    /**
     * Returns whether this map is empty.
     *
     * @return {@code true} if this map has no elements, {@code false}
     *         otherwise.
     * @see #size()
     */
    @Override public boolean isEmpty() {
        return size == 0;
    }

    /**
     * Returns the number of elements in this map.
     *
     * @return the number of elements in this map.
     */
    @Override public int size() {
        return size;
    }

    /**
     * Returns the value of the mapping with the specified key.
     *
     * @param key
     *            the key.
     * @return the value of the mapping with the specified key, or {@code null}
     *         if no mapping for the specified key is found.
     */
    public V get(Object key) {
        if (key == null) {
            HashMapEntry<K, V> e = entryForNullKey;
            return e == null ? null : e.value;
        }

        // Doug Lea's supplemental secondaryHash function (inlined)
        int hash = key.hashCode();
        hash ^= (hash >>> 20) ^ (hash >>> 12);
        hash ^= (hash >>> 7) ^ (hash >>> 4);

        HashMapEntry<K, V>[] tab = table;
        for (HashMapEntry<K, V> e = tab[hash & (tab.length - 1)]; e != null; e = e.next) 
        {
            K eKey = e.key;
            if (eKey == key || (e.hash == hash && key.equals(eKey))) {
                return e.value;
            }
        }
        return null;
    }

    /**
     * Returns whether this map contains the specified key.
     *
     * @param key
     *            the key to search for.
     * @return {@code true} if this map contains the specified key,
     *         {@code false} otherwise.
     */
    @Override public boolean containsKey(Object key) {
        if (key == null) {
            return entryForNullKey != null;
        }

        // Doug Lea's supplemental secondaryHash function (inlined)
        int hash = key.hashCode();
        hash ^= (hash >>> 20) ^ (hash >>> 12);
        hash ^= (hash >>> 7) ^ (hash >>> 4);

        HashMapEntry<K, V>[] tab = table;
        for (HashMapEntry<K, V> e = tab[hash & (tab.length - 1)];
                e != null; e = e.next) {
            K eKey = e.key;
            if (eKey == key || (e.hash == hash && key.equals(eKey))) {
                return true;
            }
        }
        return false;
    }

    /**
     * Returns whether this map contains the specified value.
     *
     * @param value
     *            the value to search for.
     * @return {@code true} if this map contains the specified value,
     *         {@code false} otherwise.
     */
    @Override public boolean containsValue(Object value) {
        HashMapEntry[] tab = table;
        int len = tab.length;
        if (value == null) {
            for (int i = 0; i < len; i++) {
                for (HashMapEntry e = tab[i]; e != null; e = e.next) {
                    if (e.value == null) {
                        return true;
                    }
                }
            }
            return entryForNullKey != null && entryForNullKey.value == null;
        }

        // value is non-null
        for (int i = 0; i < len; i++) {
            for (HashMapEntry e = tab[i]; e != null; e = e.next) {
                if (value.equals(e.value)) {
                    return true;
                }
            }
        }
        return entryForNullKey != null && value.equals(entryForNullKey.value);
    }

    /**
     * Maps the specified key to the specified value.
     *
     * @param key
     *            the key.
     * @param value
     *            the value.
     * @return the value of any previous mapping with the specified key or
     *         {@code null} if there was no such mapping.
     */
    @Override public V put(K key, V value) {
        if (key == null) {
            return putValueForNullKey(value);
        }

        int hash = secondaryHash(key.hashCode());
        HashMapEntry<K, V>[] tab = table;
        int index = hash & (tab.length - 1);
        for (HashMapEntry<K, V> e = tab[index]; e != null; e = e.next) {
            if (e.hash == hash && key.equals(e.key)) {
                preModify(e);
                V oldValue = e.value;
                e.value = value;
                return oldValue;
            }
        }

        // No entry for (non-null) key is present; create one
        modCount++;
        if (size++ > threshold) {
            tab = doubleCapacity();
            index = hash & (tab.length - 1);
        }
        addNewEntry(key, value, hash, index);
        return null;
    }

    private V putValueForNullKey(V value) {
        HashMapEntry<K, V> entry = entryForNullKey;
        if (entry == null) {
            addNewEntryForNullKey(value);
            size++;
            modCount++;
            return null;
        } else {
            preModify(entry);
            V oldValue = entry.value;
            entry.value = value;
            return oldValue;
        }
    }

    /**
     * Give LinkedHashMap a chance to take action when we modify an existing
     * entry.
     *
     * @param e the entry we're about to modify.
     */
    void preModify(HashMapEntry<K, V> e) { }

    /**
     * This method is just like put, except that it doesn't do things that
     * are inappropriate or unnecessary for constructors and pseudo-constructors
     * (i.e., clone, readObject). In particular, this method does not check to
     * ensure that capacity is sufficient, and does not increment modCount.
     */
    private void constructorPut(K key, V value) {
        if (key == null) {
            HashMapEntry<K, V> entry = entryForNullKey;
            if (entry == null) {
                entryForNullKey = constructorNewEntry(null, value, 0, null);
                size++;
            } else {
                entry.value = value;
            }
            return;
        }

        int hash = secondaryHash(key.hashCode());
        HashMapEntry<K, V>[] tab = table;
        int index = hash & (tab.length - 1);
        HashMapEntry<K, V> first = tab[index];
        for (HashMapEntry<K, V> e = first; e != null; e = e.next) {
            if (e.hash == hash && key.equals(e.key)) {
                e.value = value;
                return;
            }
        }

        // No entry for (non-null) key is present; create one
        tab[index] = constructorNewEntry(key, value, hash, first);
        size++;
    }

    /**
     * Creates a new entry for the given key, value, hash, and index and
     * inserts it into the hash table. This method is called by put
     * (and indirectly, putAll), and overridden by LinkedHashMap. The hash
     * must incorporate the secondary hash function.
     */
    void addNewEntry(K key, V value, int hash, int index) {
        table[index] = new HashMapEntry<K, V>(key, value, hash, table[index]);
    }

    /**
     * Creates a new entry for the null key, and the given value and
     * inserts it into the hash table. This method is called by put
     * (and indirectly, putAll), and overridden by LinkedHashMap.
     */
    void addNewEntryForNullKey(V value) {
        entryForNullKey = new HashMapEntry<K, V>(null, value, 0, null);
    }

    /**
     * Like newEntry, but does not perform any activity that would be
     * unnecessary or inappropriate for constructors. In this class, the
     * two methods behave identically; in LinkedHashMap, they differ.
     */
    HashMapEntry<K, V> constructorNewEntry(
            K key, V value, int hash, HashMapEntry<K, V> first) {
        return new HashMapEntry<K, V>(key, value, hash, first);
    }

    /**
     * Copies all the mappings in the specified map to this map. These mappings
     * will replace all mappings that this map had for any of the keys currently
     * in the given map.
     *
     * @param map
     *            the map to copy mappings from.
     */
    @Override public void putAll(Map<? extends K, ? extends V> map) {
        ensureCapacity(map.size());
        super.putAll(map);
    }

    /**
     * Ensures that the hash table has sufficient capacity to store the
     * specified number of mappings, with room to grow. If not, it increases the
     * capacity as appropriate. Like doubleCapacity, this method moves existing
     * entries to new buckets as appropriate. Unlike doubleCapacity, this method
     * can grow the table by factors of 2^n for n > 1. Hopefully, a single call
     * to this method will be faster than multiple calls to doubleCapacity.
     *
     *  <p>This method is called only by putAll.
     */
    private void ensureCapacity(int numMappings) {
        int newCapacity = roundUpToPowerOfTwo(capacityForInitSize(numMappings));
        HashMapEntry<K, V>[] oldTable = table;
        int oldCapacity = oldTable.length;
        if (newCapacity <= oldCapacity) {
            return;
        }
        if (newCapacity == oldCapacity * 2) {
            doubleCapacity();
            return;
        }

        // We're growing by at least 4x, rehash in the obvious way
        HashMapEntry<K, V>[] newTable = makeTable(newCapacity);
        if (size != 0) {
            int newMask = newCapacity - 1;
            for (int i = 0; i < oldCapacity; i++) {
                for (HashMapEntry<K, V> e = oldTable[i]; e != null;) {
                    HashMapEntry<K, V> oldNext = e.next;
                    int newIndex = e.hash & newMask;
                    HashMapEntry<K, V> newNext = newTable[newIndex];
                    newTable[newIndex] = e;
                    e.next = newNext;
                    e = oldNext;
                }
            }
        }
    }

    /**
     * Allocate a table of the given capacity and set the threshold accordingly.
     * @param newCapacity must be a power of two
     */
    private HashMapEntry<K, V>[] makeTable(int newCapacity) {
        @SuppressWarnings("unchecked") HashMapEntry<K, V>[] newTable
                = (HashMapEntry<K, V>[]) new HashMapEntry[newCapacity];
        table = newTable;
        threshold = (newCapacity >> 1) + (newCapacity >> 2); // 3/4 capacity
        return newTable;
    }

    /**
     * Doubles the capacity of the hash table. Existing entries are placed in
     * the correct bucket on the enlarged table. If the current capacity is,
     * MAXIMUM_CAPACITY, this method is a no-op. Returns the table, which
     * will be new unless we were already at MAXIMUM_CAPACITY.
     */
    private HashMapEntry<K, V>[] doubleCapacity() {
        HashMapEntry<K, V>[] oldTable = table;
        int oldCapacity = oldTable.length;
        if (oldCapacity == MAXIMUM_CAPACITY) {
            return oldTable;
        }
        int newCapacity = oldCapacity * 2;
        HashMapEntry<K, V>[] newTable = makeTable(newCapacity);
        if (size == 0) {
            return newTable;
        }

        for (int j = 0; j < oldCapacity; j++) {
            /*
             * Rehash the bucket using the minimum number of field writes.
             * This is the most subtle and delicate code in the class.
             */
            HashMapEntry<K, V> e = oldTable[j];
            if (e == null) {
                continue;
            }
            int highBit = e.hash & oldCapacity;
            HashMapEntry<K, V> broken = null;
            newTable[j | highBit] = e;
            for (HashMapEntry<K, V> n = e.next; n != null; e = n, n = n.next) {
                int nextHighBit = n.hash & oldCapacity;
                if (nextHighBit != highBit) {
                    if (broken == null)
                        newTable[j | nextHighBit] = n;
                    else
                        broken.next = n;
                    broken = e;
                    highBit = nextHighBit;
                }
            }
            if (broken != null)
                broken.next = null;
        }
        return newTable;
    }

    /**
     * Removes the mapping with the specified key from this map.
     *
     * @param key
     *            the key of the mapping to remove.
     * @return the value of the removed mapping or {@code null} if no mapping
     *         for the specified key was found.
     */
    @Override public V remove(Object key) {
        if (key == null) {
            return removeNullKey();
        }
        int hash = secondaryHash(key.hashCode());
        HashMapEntry<K, V>[] tab = table;
        int index = hash & (tab.length - 1);
        for (HashMapEntry<K, V> e = tab[index], prev = null;
                e != null; prev = e, e = e.next) {
            if (e.hash == hash && key.equals(e.key)) {
                if (prev == null) {
                    tab[index] = e.next;
                } else {
                    prev.next = e.next;
                }
                modCount++;
                size--;
                postRemove(e);
                return e.value;
            }
        }
        return null;
    }

    private V removeNullKey() {
        HashMapEntry<K, V> e = entryForNullKey;
        if (e == null) {
            return null;
        }
        entryForNullKey = null;
        modCount++;
        size--;
        postRemove(e);
        return e.value;
    }

    /**
     * Subclass overrides this method to unlink entry.
     */
    void postRemove(HashMapEntry<K, V> e) { }

    /**
     * Removes all mappings from this hash map, leaving it empty.
     *
     * @see #isEmpty
     * @see #size
     */
    @Override public void clear() {
        if (size != 0) {
            Arrays.fill(table, null);
            entryForNullKey = null;
            modCount++;
            size = 0;
        }
    }

    /**
     * Returns a set of the keys contained in this map. The set is backed by
     * this map so changes to one are reflected by the other. The set does not
     * support adding.
     *
     * @return a set of the keys.
     */
    @Override public Set<K> keySet() {
        Set<K> ks = keySet;
        return (ks != null) ? ks : (keySet = new KeySet());
    }

    /**
     * Returns a collection of the values contained in this map. The collection
     * is backed by this map so changes to one are reflected by the other. The
     * collection supports remove, removeAll, retainAll and clear operations,
     * and it does not support add or addAll operations.
     * <p>
     * This method returns a collection which is the subclass of
     * AbstractCollection. The iterator method of this subclass returns a
     * "wrapper object" over the iterator of map's entrySet(). The {@code size}
     * method wraps the map's size method and the {@code contains} method wraps
     * the map's containsValue method.
     * </p>
     * <p>
     * The collection is created when this method is called for the first time
     * and returned in response to all subsequent calls. This method may return
     * different collections when multiple concurrent calls occur, since no
     * synchronization is performed.
     * </p>
     *
     * @return a collection of the values contained in this map.
     */
    @Override public Collection<V> values() {
        Collection<V> vs = values;
        return (vs != null) ? vs : (values = new Values());
    }

    /**
     * Returns a set containing all of the mappings in this map. Each mapping is
     * an instance of {@link Map.Entry}. As the set is backed by this map,
     * changes in one will be reflected in the other.
     *
     * @return a set of the mappings.
     */
    public Set<Entry<K, V>> entrySet() {
        Set<Entry<K, V>> es = entrySet;
        return (es != null) ? es : (entrySet = new EntrySet());
    }

    static class HashMapEntry<K, V> implements Entry<K, V> {
        final K key;
        V value;
        final int hash;
        HashMapEntry<K, V> next;

        HashMapEntry(K key, V value, int hash, HashMapEntry<K, V> next) {
            this.key = key;
            this.value = value;
            this.hash = hash;
            this.next = next;
        }

        public final K getKey() {
            return key;
        }

        public final V getValue() {
            return value;
        }

        public final V setValue(V value) {
            V oldValue = this.value;
            this.value = value;
            return oldValue;
        }

        @Override public final boolean equals(Object o) {
            if (!(o instanceof Entry)) {
                return false;
            }
            Entry<?, ?> e = (Entry<?, ?>) o;
            return Objects.equal(e.getKey(), key)
                    && Objects.equal(e.getValue(), value);
        }

        @Override public final int hashCode() {
            return (key == null ? 0 : key.hashCode()) ^
                    (value == null ? 0 : value.hashCode());
        }

        @Override public final String toString() {
            return key + "=" + value;
        }
    }

    private abstract class HashIterator {
        int nextIndex;
        HashMapEntry<K, V> nextEntry = entryForNullKey;
        HashMapEntry<K, V> lastEntryReturned;
        int expectedModCount = modCount;

        HashIterator() {
            if (nextEntry == null) {
                HashMapEntry<K, V>[] tab = table;
                HashMapEntry<K, V> next = null;
                while (next == null && nextIndex < tab.length) {
                    next = tab[nextIndex++];
                }
                nextEntry = next;
            }
        }

        public boolean hasNext() {
            return nextEntry != null;
        }

        HashMapEntry<K, V> nextEntry() {
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
            if (nextEntry == null)
                throw new NoSuchElementException();

            HashMapEntry<K, V> entryToReturn = nextEntry;
            HashMapEntry<K, V>[] tab = table;
            HashMapEntry<K, V> next = entryToReturn.next;
            while (next == null && nextIndex < tab.length) {
                next = tab[nextIndex++];
            }
            nextEntry = next;
            return lastEntryReturned = entryToReturn;
        }

        public void remove() {
            if (lastEntryReturned == null)
                throw new IllegalStateException();
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
            HashMap.this.remove(lastEntryReturned.key);
            lastEntryReturned = null;
            expectedModCount = modCount;
        }
    }

    private final class KeyIterator extends HashIterator
            implements Iterator<K> {
        public K next() { return nextEntry().key; }
    }

    private final class ValueIterator extends HashIterator
            implements Iterator<V> {
        public V next() { return nextEntry().value; }
    }

    private final class EntryIterator extends HashIterator
            implements Iterator<Entry<K, V>> {
        public Entry<K, V> next() { return nextEntry(); }
    }

    /**
     * Returns true if this map contains the specified mapping.
     */
    private boolean containsMapping(Object key, Object value) {
        if (key == null) {
            HashMapEntry<K, V> e = entryForNullKey;
            return e != null && Objects.equal(value, e.value);
        }

        int hash = secondaryHash(key.hashCode());
        HashMapEntry<K, V>[] tab = table;
        int index = hash & (tab.length - 1);
        for (HashMapEntry<K, V> e = tab[index]; e != null; e = e.next) {
            if (e.hash == hash && key.equals(e.key)) {
                return Objects.equal(value, e.value);
            }
        }
        return false; // No entry for key
    }

    /**
     * Removes the mapping from key to value and returns true if this mapping
     * exists; otherwise, returns does nothing and returns false.
     */
    private boolean removeMapping(Object key, Object value) {
        if (key == null) {
            HashMapEntry<K, V> e = entryForNullKey;
            if (e == null || !Objects.equal(value, e.value)) {
                return false;
            }
            entryForNullKey = null;
            modCount++;
            size--;
            postRemove(e);
            return true;
        }

        int hash = secondaryHash(key.hashCode());
        HashMapEntry<K, V>[] tab = table;
        int index = hash & (tab.length - 1);
        for (HashMapEntry<K, V> e = tab[index], prev = null;
                e != null; prev = e, e = e.next) {
            if (e.hash == hash && key.equals(e.key)) {
                if (!Objects.equal(value, e.value)) {
                    return false;  // Map has wrong value for key
                }
                if (prev == null) {
                    tab[index] = e.next;
                } else {
                    prev.next = e.next;
                }
                modCount++;
                size--;
                postRemove(e);
                return true;
            }
        }
        return false; // No entry for key
    }

    // Subclass (LinkedHashMap) overrides these for correct iteration order
    Iterator<K> newKeyIterator() { return new KeyIterator();   }
    Iterator<V> newValueIterator() { return new ValueIterator(); }
    Iterator<Entry<K, V>> newEntryIterator() { return new EntryIterator(); }

    private final class KeySet extends AbstractSet<K> {
        public Iterator<K> iterator() {
            return newKeyIterator();
        }
        public int size() {
            return size;
        }
        public boolean isEmpty() {
            return size == 0;
        }
        public boolean contains(Object o) {
            return containsKey(o);
        }
        public boolean remove(Object o) {
            int oldSize = size;
            HashMap.this.remove(o);
            return size != oldSize;
        }
        public void clear() {
            HashMap.this.clear();
        }
    }

    private final class Values extends AbstractCollection<V> {
        public Iterator<V> iterator() {
            return newValueIterator();
        }
        public int size() {
            return size;
        }
        public boolean isEmpty() {
            return size == 0;
        }
        public boolean contains(Object o) {
            return containsValue(o);
        }
        public void clear() {
            HashMap.this.clear();
        }
    }

    private final class EntrySet extends AbstractSet<Entry<K, V>> {
        public Iterator<Entry<K, V>> iterator() {
            return newEntryIterator();
        }
        public boolean contains(Object o) {
            if (!(o instanceof Entry))
                return false;
            Entry<?, ?> e = (Entry<?, ?>) o;
            return containsMapping(e.getKey(), e.getValue());
        }
        public boolean remove(Object o) {
            if (!(o instanceof Entry))
                return false;
            Entry<?, ?> e = (Entry<?, ?>)o;
            return removeMapping(e.getKey(), e.getValue());
        }
        public int size() {
            return size;
        }
        public boolean isEmpty() {
            return size == 0;
        }
        public void clear() {
            HashMap.this.clear();
        }
    }

    /**
     * Applies a supplemental hash function to a given hashCode, which defends
     * against poor quality hash functions. This is critical because HashMap
     * uses power-of-two length hash tables, that otherwise encounter collisions
     * for hashCodes that do not differ in lower or upper bits.
     */
    private static int secondaryHash(int h) {
        // Doug Lea's supplemental hash function
        h ^= (h >>> 20) ^ (h >>> 12);
        return h ^ (h >>> 7) ^ (h >>> 4);
    }

    /**
     * Returns the smallest power of two >= its argument, with several caveats:
     * If the argument is negative but not Integer.MIN_VALUE, the method returns
     * zero. If the argument is > 2^30 or equal to Integer.MIN_VALUE, the method
     * returns Integer.MIN_VALUE. If the argument is zero, the method returns
     * zero.
     */
    private static int roundUpToPowerOfTwo(int i) {
        i--; // If input is a power of two, shift its high-order bit right

        // "Smear" the high-order bit all the way to the right
        i |= i >>>  1;
        i |= i >>>  2;
        i |= i >>>  4;
        i |= i >>>  8;
        i |= i >>> 16;

        return i + 1;
    }

    private static final long serialVersionUID = 362498820763181265L;

    private static final ObjectStreamField[] serialPersistentFields = {
        new ObjectStreamField("loadFactor", float.class)
    };

    private void writeObject(ObjectOutputStream stream) throws IOException {
        // Emulate loadFactor field for other implementations to read
        ObjectOutputStream.PutField fields = stream.putFields();
        fields.put("loadFactor", DEFAULT_LOAD_FACTOR);
        stream.writeFields();

        stream.writeInt(table.length); // Capacity
        stream.writeInt(size);
        for (Entry<K, V> e : entrySet()) {
            stream.writeObject(e.getKey());
            stream.writeObject(e.getValue());
        }
    }

    private void readObject(ObjectInputStream stream) throws IOException,
            ClassNotFoundException {
        stream.defaultReadObject();
        int capacity = stream.readInt();
        if (capacity < 0) {
            throw new InvalidObjectException("Capacity: " + capacity);
        }
        if (capacity < MINIMUM_CAPACITY) {
            capacity = MINIMUM_CAPACITY;
        } else if (capacity > MAXIMUM_CAPACITY) {
            capacity = MAXIMUM_CAPACITY;
        } else {
            capacity = roundUpToPowerOfTwo(capacity);
        }
        makeTable(capacity);

        int size = stream.readInt();
        if (size < 0) {
            throw new InvalidObjectException("Size: " + size);
        }

        init(); // Give subclass (LinkedHashMap) a chance to initialize itself
        for (int i = 0; i < size; i++) {
            @SuppressWarnings("unchecked") K key = (K) stream.readObject();
            @SuppressWarnings("unchecked") V val = (V) stream.readObject();
            constructorPut(key, val);
        }
    }
}

TreeMap

package java.util;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectInputStream.GetField;
import java.io.ObjectOutputStream;
import java.io.ObjectStreamException;
import java.io.Serializable;
import static java.util.TreeMap.Bound.*;
import static java.util.TreeMap.Relation.*;
import libcore.util.Objects;
/**
 * A map whose entries are sorted by their keys. All optional operations such as
 * {@link #put} and {@link #remove} are supported.
 *
 * <p>This map sorts keys using either a user-supplied comparator or the key's
 * natural order:
 * <ul>
 *   <li>User supplied comparators must be able to compare any pair of keys in
 *       this map. If a user-supplied comparator is in use, it will be returned
 *       by {@link #comparator}.
 *   <li>If no user-supplied comparator is supplied, keys will be sorted by
 *       their natural order. Keys must be <i>mutually comparable</i>: they must
 *       implement {@link Comparable} and {@link Comparable#compareTo
 *       compareTo()} must be able to compare each key with any other key in
 *       this map. In this case {@link #comparator} will return null.
 * </ul>
 * With either a comparator or a natural ordering, comparisons should be
 * <i>consistent with equals</i>. An ordering is consistent with equals if for
 * every pair of keys {@code a} and {@code b}, {@code a.equals(b)} if and only
 * if {@code compare(a, b) == 0}.
 *
 * <p>When the ordering is not consistent with equals the behavior of this
 * class is well defined but does not honor the contract specified by {@link
 * Map}. Consider a tree map of case-insensitive strings, an ordering that is
 * not consistent with equals: <pre>   {@code
 *   TreeMap<String, String> map = new TreeMap<String, String>(String.CASE_INSENSITIVE_ORDER);
 *   map.put("a", "android");
 *
 *   // The Map API specifies that the next line should print "null" because
 *   // "a".equals("A") is false and there is no mapping for upper case "A".
 *   // But the case insensitive ordering says compare("a", "A") == 0. TreeMap
 *   // uses only comparators/comparable on keys and so this prints "android".
 *   System.out.println(map.get("A"));
 * }</pre>
 *
 * @since 1.2
 */
public class TreeMap<K, V> extends AbstractMap<K, V>
        implements SortedMap<K, V>, NavigableMap<K, V>, Cloneable, Serializable {

    @SuppressWarnings("unchecked") // to avoid Comparable<Comparable<Comparable<...>>>
    private static final Comparator<Comparable> NATURAL_ORDER = new Comparator<Comparable>() {
        public int compare(Comparable a, Comparable b) {
            return a.compareTo(b);
        }
    };

    Comparator<? super K> comparator;
    Node<K, V> root;
    int size = 0;
    int modCount = 0;

    /**
     * Create a natural order, empty tree map whose keys must be mutually
     * comparable and non-null.
     */
    @SuppressWarnings("unchecked") // unsafe! this assumes K is comparable
    public TreeMap() {
        this.comparator = (Comparator<? super K>) NATURAL_ORDER;
    }

    /**
     * Create a natural order tree map populated with the key/value pairs of
     * {@code copyFrom}. This map's keys must be mutually comparable and
     * non-null.
     *
     * <p>Even if {@code copyFrom} is a {@code SortedMap}, the constructed map
     * <strong>will not</strong> use {@code copyFrom}'s ordering. This
     * constructor always creates a naturally-ordered map. Because the {@code
     * TreeMap} constructor overloads are ambiguous, prefer to construct a map
     * and populate it in two steps: <pre>   {@code
     *   TreeMap<String, Integer> customOrderedMap
     *       = new TreeMap<String, Integer>(copyFrom.comparator());
     *   customOrderedMap.putAll(copyFrom);
     * }</pre>
     */
    public TreeMap(Map<? extends K, ? extends V> copyFrom) {
        this();
        for (Map.Entry<? extends K, ? extends V> entry : copyFrom.entrySet()) {
            putInternal(entry.getKey(), entry.getValue());
        }
    }

    /**
     * Create a tree map ordered by {@code comparator}. This map's keys may only
     * be null if {@code comparator} permits.
     *
     * @param comparator the comparator to order elements with, or {@code null} to use the natural
     * ordering.
     */
    @SuppressWarnings("unchecked") // unsafe! if comparator is null, this assumes K is comparable
    public TreeMap(Comparator<? super K> comparator) {
        if (comparator != null) {
            this.comparator = comparator;
        } else {
            this.comparator = (Comparator<? super K>) NATURAL_ORDER;
        }
    }

    /**
     * Create a tree map with the ordering and key/value pairs of {@code
     * copyFrom}. This map's keys may only be null if the {@code copyFrom}'s
     * ordering permits.
     *
     * <p>The constructed map <strong>will always use</strong> {@code
     * copyFrom}'s ordering. Because the {@code TreeMap} constructor overloads
     * are ambigous, prefer to construct a map and populate it in two steps:
     * <pre>   {@code
     *   TreeMap<String, Integer> customOrderedMap
     *       = new TreeMap<String, Integer>(copyFrom.comparator());
     *   customOrderedMap.putAll(copyFrom);
     * }</pre>
     */
    @SuppressWarnings("unchecked") // if copyFrom's keys are comparable this map's keys must be also
    public TreeMap(SortedMap<K, ? extends V> copyFrom) {
        Comparator<? super K> sourceComparator = copyFrom.comparator();
        if (sourceComparator != null) {
            this.comparator = sourceComparator;
        } else {
            this.comparator = (Comparator<? super K>) NATURAL_ORDER;
        }
        for (Map.Entry<K, ? extends V> entry : copyFrom.entrySet()) {
            putInternal(entry.getKey(), entry.getValue());
        }
    }

    @Override public Object clone() {
        try {
            @SuppressWarnings("unchecked") // super.clone() must return the same type
            TreeMap<K, V> map = (TreeMap<K, V>) super.clone();
            map.root = root != null ? root.copy(null) : null;
            map.entrySet = null;
            map.keySet = null;
            return map;
        } catch (CloneNotSupportedException e) {
            throw new AssertionError();
        }
    }

    @Override public int size() {
        return size;
    }

    @Override public boolean isEmpty() {
        return size == 0;
    }

    @Override public V get(Object key) {
        Entry<K, V> entry = findByObject(key);
        return entry != null ? entry.getValue() : null;
    }

    @Override public boolean containsKey(Object key) {
        return findByObject(key) != null;
    }

    @Override public V put(K key, V value) {
        return putInternal(key, value);
    }

    @Override public void clear() {
        root = null;
        size = 0;
        modCount++;
    }

    @Override public V remove(Object key) {
        Node<K, V> node = removeInternalByKey(key);
        return node != null ? node.value : null;
    }

    /*
     * AVL methods
     */

    enum Relation {
        LOWER,
        FLOOR,
        EQUAL,
        CREATE,
        CEILING,
        HIGHER;

        /**
         * Returns a possibly-flipped relation for use in descending views.
         *
         * @param ascending false to flip; true to return this.
         */
        Relation forOrder(boolean ascending) {
            if (ascending) {
                return this;
            }

            switch (this) {
                case LOWER:
                    return HIGHER;
                case FLOOR:
                    return CEILING;
                case EQUAL:
                    return EQUAL;
                case CEILING:
                    return FLOOR;
                case HIGHER:
                    return LOWER;
                default:
                    throw new IllegalStateException();
            }
        }
    }

    V putInternal(K key, V value) {
        Node<K, V> created = find(key, Relation.CREATE);
        V result = created.value;
        created.value = value;
        return result;
    }

    /**
     * Returns the node at or adjacent to the given key, creating it if requested.
     *
     * @throws ClassCastException if {@code key} and the tree's keys aren't mutually comparable.
     */
    Node<K, V> find(K key, Relation relation) {
        if (root == null) {
            if (comparator == NATURAL_ORDER && !(key instanceof Comparable)) {
                throw new ClassCastException(key.getClass().getName() + " is not Comparable"); // NullPointerException ok
            }
            if (relation == Relation.CREATE) {
                root = new Node<K, V>(null, key);
                size = 1;
                modCount++;
                return root;
            } else {
                return null;
            }
        }

        /*
         * Micro-optimization: avoid polymorphic calls to Comparator.compare().
         * This is 10% faster for naturally ordered trees.
         */
        @SuppressWarnings("unchecked") // will throw a ClassCastException below if there's trouble
        Comparable<Object> comparableKey = (comparator == NATURAL_ORDER)
                ? (Comparable<Object>) key
                : null;

        Node<K, V> nearest = root;
        while (true) {
            int comparison = (comparableKey != null)
                    ? comparableKey.compareTo(nearest.key)
                    : comparator.compare(key, nearest.key);

            /*
             * We found the requested key.
             */
            if (comparison == 0) {
                switch (relation) {
                    case LOWER:
                        return nearest.prev();
                    case FLOOR:
                    case EQUAL:
                    case CREATE:
                    case CEILING:
                        return nearest;
                    case HIGHER:
                        return nearest.next();
                }
            }

            Node<K, V> child = (comparison < 0) ? nearest.left : nearest.right;
            if (child != null) {
                nearest = child;
                continue;
            }

            /*
             * We found a nearest node. Every key not in the tree has up to two
             * nearest nodes, one lower and one higher.
             */

            if (comparison < 0) { // nearest.key is higher
                switch (relation) {
                    case LOWER:
                    case FLOOR:
                        return nearest.prev();
                    case CEILING:
                    case HIGHER:
                        return nearest;
                    case EQUAL:
                        return null;
                    case CREATE:
                        Node<K, V> created = new Node<K, V>(nearest, key);
                        nearest.left = created;
                        size++;
                        modCount++;
                        rebalance(nearest, true);
                        return created;
                }
            } else { // comparison > 0, nearest.key is lower
                switch (relation) {
                    case LOWER:
                    case FLOOR:
                        return nearest;
                    case CEILING:
                    case HIGHER:
                        return nearest.next();
                    case EQUAL:
                        return null;
                    case CREATE:
                        Node<K, V> created = new Node<K, V>(nearest, key);
                        nearest.right = created;
                        size++;
                        modCount++;
                        rebalance(nearest, true);
                        return created;
                }
            }
        }
    }

    @SuppressWarnings("unchecked") // this method throws ClassCastExceptions!
    Node<K, V> findByObject(Object key) {
        return find((K) key, EQUAL);
    }

    /**
     * Returns this map's entry that has the same key and value as {@code
     * entry}, or null if this map has no such entry.
     *
     * <p>This method uses the comparator for key equality rather than {@code
     * equals}. If this map's comparator isn't consistent with equals (such as
     * {@code String.CASE_INSENSITIVE_ORDER}), then {@code remove()} and {@code
     * contains()} will violate the collections API.
     */
    Node<K, V> findByEntry(Entry<?, ?> entry) {
        Node<K, V> mine = findByObject(entry.getKey());
        boolean valuesEqual = mine != null && Objects.equal(mine.value, entry.getValue());
        return valuesEqual ? mine : null;
    }

    /**
     * Removes {@code node} from this tree, rearranging the tree's structure as
     * necessary.
     */
    void removeInternal(Node<K, V> node) {
        Node<K, V> left = node.left;
        Node<K, V> right = node.right;
        Node<K, V> originalParent = node.parent;
        if (left != null && right != null) {

            /*
             * To remove a node with both left and right subtrees, move an
             * adjacent node from one of those subtrees into this node's place.
             *
             * Removing the adjacent node may change this node's subtrees. This
             * node may no longer have two subtrees once the adjacent node is
             * gone!
             */

            Node<K, V> adjacent = (left.height > right.height) ? left.last() : right.first();
            removeInternal(adjacent); // takes care of rebalance and size--

            int leftHeight = 0;
            left = node.left;
            if (left != null) {
                leftHeight = left.height;
                adjacent.left = left;
                left.parent = adjacent;
                node.left = null;
            }
            int rightHeight = 0;
            right = node.right;
            if (right != null) {
                rightHeight = right.height;
                adjacent.right = right;
                right.parent = adjacent;
                node.right = null;
            }
            adjacent.height = Math.max(leftHeight, rightHeight) + 1;
            replaceInParent(node, adjacent);
            return;
        } else if (left != null) {
            replaceInParent(node, left);
            node.left = null;
        } else if (right != null) {
            replaceInParent(node, right);
            node.right = null;
        } else {
            replaceInParent(node, null);
        }

        rebalance(originalParent, false);
        size--;
        modCount++;
    }

    Node<K, V> removeInternalByKey(Object key) {
        Node<K, V> node = findByObject(key);
        if (node != null) {
            removeInternal(node);
        }
        return node;
    }

    private void replaceInParent(Node<K, V> node, Node<K, V> replacement) {
        Node<K, V> parent = node.parent;
        node.parent = null;
        if (replacement != null) {
            replacement.parent = parent;
        }

        if (parent != null) {
            if (parent.left == node) {
                parent.left = replacement;
            } else {
                assert (parent.right == node);
                parent.right = replacement;
            }
        } else {
            root = replacement;
        }
    }

    /**
     * Rebalances the tree by making any AVL rotations necessary between the
     * newly-unbalanced node and the tree's root.
     *
     * @param insert true if the node was unbalanced by an insert; false if it
     *     was by a removal.
     */
    private void rebalance(Node<K, V> unbalanced, boolean insert) {
        for (Node<K, V> node = unbalanced; node != null; node = node.parent) {
            Node<K, V> left = node.left;
            Node<K, V> right = node.right;
            int leftHeight = left != null ? left.height : 0;
            int rightHeight = right != null ? right.height : 0;

            int delta = leftHeight - rightHeight;
            if (delta == -2) {
                Node<K, V> rightLeft = right.left;
                Node<K, V> rightRight = right.right;
                int rightRightHeight = rightRight != null ? rightRight.height : 0;
                int rightLeftHeight = rightLeft != null ? rightLeft.height : 0;

                int rightDelta = rightLeftHeight - rightRightHeight;
                if (rightDelta == -1 || (rightDelta == 0 && !insert)) {
                    rotateLeft(node); // AVL right right
                } else {
                    assert (rightDelta == 1);
                    rotateRight(right); // AVL right left
                    rotateLeft(node);
                }
                if (insert) {
                    break; // no further rotations will be necessary
                }

            } else if (delta == 2) {
                Node<K, V> leftLeft = left.left;
                Node<K, V> leftRight = left.right;
                int leftRightHeight = leftRight != null ? leftRight.height : 0;
                int leftLeftHeight = leftLeft != null ? leftLeft.height : 0;

                int leftDelta = leftLeftHeight - leftRightHeight;
                if (leftDelta == 1 || (leftDelta == 0 && !insert)) {
                    rotateRight(node); // AVL left left
                } else {
                    assert (leftDelta == -1);
                    rotateLeft(left); // AVL left right
                    rotateRight(node);
                }
                if (insert) {
                    break; // no further rotations will be necessary
                }

            } else if (delta == 0) {
                node.height = leftHeight + 1; // leftHeight == rightHeight
                if (insert) {
                    break; // the insert caused balance, so rebalancing is done!
                }

            } else {
                assert (delta == -1 || delta == 1);
                node.height = Math.max(leftHeight, rightHeight) + 1;
                if (!insert) {
                    break; // the height hasn't changed, so rebalancing is done!
                }
            }
        }
    }

    /**
     * Rotates the subtree so that its root's right child is the new root.
     */
    private void rotateLeft(Node<K, V> root) {
        Node<K, V> left = root.left;
        Node<K, V> pivot = root.right;
        Node<K, V> pivotLeft = pivot.left;
        Node<K, V> pivotRight = pivot.right;

        // move the pivot's left child to the root's right
        root.right = pivotLeft;
        if (pivotLeft != null) {
            pivotLeft.parent = root;
        }

        replaceInParent(root, pivot);

        // move the root to the pivot's left
        pivot.left = root;
        root.parent = pivot;

        // fix heights
        root.height = Math.max(left != null ? left.height : 0,
                pivotLeft != null ? pivotLeft.height : 0) + 1;
        pivot.height = Math.max(root.height,
                pivotRight != null ? pivotRight.height : 0) + 1;
    }

    /**
     * Rotates the subtree so that its root's left child is the new root.
     */
    private void rotateRight(Node<K, V> root) {
        Node<K, V> pivot = root.left;
        Node<K, V> right = root.right;
        Node<K, V> pivotLeft = pivot.left;
        Node<K, V> pivotRight = pivot.right;

        // move the pivot's right child to the root's left
        root.left = pivotRight;
        if (pivotRight != null) {
            pivotRight.parent = root;
        }

        replaceInParent(root, pivot);

        // move the root to the pivot's right
        pivot.right = root;
        root.parent = pivot;

        // fixup heights
        root.height = Math.max(right != null ? right.height : 0,
                pivotRight != null ? pivotRight.height : 0) + 1;
        pivot.height = Math.max(root.height,
                pivotLeft != null ? pivotLeft.height : 0) + 1;
    }

    /*
     * Navigable methods.
     */

    /**
     * Returns an immutable version of {@param entry}. Need this because we allow entry to be null,
     * in which case we return a null SimpleImmutableEntry.
     */
    private SimpleImmutableEntry<K, V> immutableCopy(Entry<K, V> entry) {
        return entry == null ? null : new SimpleImmutableEntry<K, V>(entry);
    }

    public Entry<K, V> firstEntry() {
        return immutableCopy(root == null ? null : root.first());
    }

    private Entry<K, V> internalPollFirstEntry() {
        if (root == null) {
            return null;
        }
        Node<K, V> result = root.first();
        removeInternal(result);
        return result;
    }

    public Entry<K, V> pollFirstEntry() {
        return immutableCopy(internalPollFirstEntry());
    }

    public K firstKey() {
        if (root == null) {
            throw new NoSuchElementException();
        }
        return root.first().getKey();
    }

    public Entry<K, V> lastEntry() {
        return immutableCopy(root == null ? null : root.last());
    }

    private Entry<K, V> internalPollLastEntry() {
        if (root == null) {
            return null;
        }
        Node<K, V> result = root.last();
        removeInternal(result);
        return result;
    }

    public Entry<K, V> pollLastEntry() {
        return immutableCopy(internalPollLastEntry());
    }

    public K lastKey() {
        if (root == null) {
            throw new NoSuchElementException();
        }
        return root.last().getKey();
    }

    public Entry<K, V> lowerEntry(K key) {
        return immutableCopy(find(key, LOWER));
    }

    public K lowerKey(K key) {
        Entry<K, V> entry = find(key, LOWER);
        return entry != null ? entry.getKey() : null;
    }

    public Entry<K, V> floorEntry(K key) {
        return immutableCopy(find(key, FLOOR));
    }

    public K floorKey(K key) {
        Entry<K, V> entry = find(key, FLOOR);
        return entry != null ? entry.getKey() : null;
    }

    public Entry<K, V> ceilingEntry(K key) {
        return immutableCopy(find(key, CEILING));
    }

    public K ceilingKey(K key) {
        Entry<K, V> entry = find(key, CEILING);
        return entry != null ? entry.getKey() : null;
    }

    public Entry<K, V> higherEntry(K key) {
        return immutableCopy(find(key, HIGHER));
    }

    public K higherKey(K key) {
        Entry<K, V> entry = find(key, HIGHER);
        return entry != null ? entry.getKey() : null;
    }

    public Comparator<? super K> comparator() {
        return comparator != NATURAL_ORDER ? comparator : null;
    }

    /*
     * View factory methods.
     */

    private EntrySet entrySet;
    private KeySet keySet;

    @Override public Set<Entry<K, V>> entrySet() {
        EntrySet result = entrySet;
        return result != null ? result : (entrySet = new EntrySet());
    }

    @Override public Set<K> keySet() {
        KeySet result = keySet;
        return result != null ? result : (keySet = new KeySet());
    }

    public NavigableSet<K> navigableKeySet() {
        KeySet result = keySet;
        return result != null ? result : (keySet = new KeySet());
    }

    public NavigableMap<K, V> subMap(K from, boolean fromInclusive, K to, boolean toInclusive) {
        Bound fromBound = fromInclusive ? INCLUSIVE : EXCLUSIVE;
        Bound toBound = toInclusive ? INCLUSIVE : EXCLUSIVE;
        return new BoundedMap(true, from, fromBound, to, toBound);
    }

    public SortedMap<K, V> subMap(K fromInclusive, K toExclusive) {
        return new BoundedMap(true, fromInclusive, INCLUSIVE, toExclusive, EXCLUSIVE);
    }

    public NavigableMap<K, V> headMap(K to, boolean inclusive) {
        Bound toBound = inclusive ? INCLUSIVE : EXCLUSIVE;
        return new BoundedMap(true, null, NO_BOUND, to, toBound);
    }

    public SortedMap<K, V> headMap(K toExclusive) {
        return new BoundedMap(true, null, NO_BOUND, toExclusive, EXCLUSIVE);
    }

    public NavigableMap<K, V> tailMap(K from, boolean inclusive) {
        Bound fromBound = inclusive ? INCLUSIVE : EXCLUSIVE;
        return new BoundedMap(true, from, fromBound, null, NO_BOUND);
    }

    public SortedMap<K, V> tailMap(K fromInclusive) {
        return new BoundedMap(true, fromInclusive, INCLUSIVE, null, NO_BOUND);
    }

    public NavigableMap<K, V> descendingMap() {
        return new BoundedMap(false, null, NO_BOUND, null, NO_BOUND);
    }

    public NavigableSet<K> descendingKeySet() {
        return new BoundedMap(false, null, NO_BOUND, null, NO_BOUND).navigableKeySet();
    }

    static class Node<K, V> implements Map.Entry<K, V> {
        Node<K, V> parent;
        Node<K, V> left;
        Node<K, V> right;
        final K key;
        V value;
        int height;

        Node(Node<K, V> parent, K key) {
            this.parent = parent;
            this.key = key;
            this.height = 1;
        }

        Node<K, V> copy(Node<K, V> parent) {
            Node<K, V> result = new Node<K, V>(parent, key);
            if (left != null) {
                result.left = left.copy(result);
            }
            if (right != null) {
                result.right = right.copy(result);
            }
            result.value = value;
            result.height = height;
            return result;
        }

        public K getKey() {
            return key;
        }

        public V getValue() {
            return value;
        }

        public V setValue(V value) {
            V oldValue = this.value;
            this.value = value;
            return oldValue;
        }

        @Override public boolean equals(Object o) {
            if (o instanceof Map.Entry) {
                Map.Entry other = (Map.Entry) o;
                return (key == null ? other.getKey() == null : key.equals(other.getKey()))
                        && (value == null ? other.getValue() == null : value.equals(other.getValue()));
            }
            return false;
        }

        @Override public int hashCode() {
            return (key == null ? 0 : key.hashCode())
                    ^ (value == null ? 0 : value.hashCode());
        }

        @Override public String toString() {
            return key + "=" + value;
        }

        /**
         * Returns the next node in an inorder traversal, or null if this is the
         * last node in the tree.
         */
        Node<K, V> next() {
            if (right != null) {
                return right.first();
            }

            Node<K, V> node = this;
            Node<K, V> parent = node.parent;
            while (parent != null) {
                if (parent.left == node) {
                    return parent;
                }
                node = parent;
                parent = node.parent;
            }
            return null;
        }

        /**
         * Returns the previous node in an inorder traversal, or null if this is
         * the first node in the tree.
         */
        public Node<K, V> prev() {
            if (left != null) {
                return left.last();
            }

            Node<K, V> node = this;
            Node<K, V> parent = node.parent;
            while (parent != null) {
                if (parent.right == node) {
                    return parent;
                }
                node = parent;
                parent = node.parent;
            }
            return null;
        }

        /**
         * Returns the first node in this subtree.
         */
        public Node<K, V> first() {
            Node<K, V> node = this;
            Node<K, V> child = node.left;
            while (child != null) {
                node = child;
                child = node.left;
            }
            return node;
        }

        /**
         * Returns the last node in this subtree.
         */
        public Node<K, V> last() {
            Node<K, V> node = this;
            Node<K, V> child = node.right;
            while (child != null) {
                node = child;
                child = node.right;
            }
            return node;
        }
    }

    /**
     * Walk the nodes of the tree left-to-right or right-to-left. Note that in
     * descending iterations, {@code next} will return the previous node.
     */
    abstract class MapIterator<T> implements Iterator<T> {
        protected Node<K, V> next;
        protected Node<K, V> last;
        protected int expectedModCount = modCount;

        MapIterator(Node<K, V> next) {
            this.next = next;
        }

        public boolean hasNext() {
            return next != null;
        }

        protected Node<K, V> stepForward() {
            if (next == null) {
                throw new NoSuchElementException();
            }
            if (modCount != expectedModCount) {
                throw new ConcurrentModificationException();
            }
            last = next;
            next = next.next();
            return last;
        }

        protected Node<K, V> stepBackward() {
            if (next == null) {
                throw new NoSuchElementException();
            }
            if (modCount != expectedModCount) {
                throw new ConcurrentModificationException();
            }
            last = next;
            next = next.prev();
            return last;
        }

        public void remove() {
            if (last == null) {
                throw new IllegalStateException();
            }
            removeInternal(last);
            expectedModCount = modCount;
            last = null;
        }
    }

    /*
     * View implementations.
     */

    class EntrySet extends AbstractSet<Map.Entry<K, V>> {
        @Override public int size() {
            return size;
        }

        @Override public Iterator<Entry<K, V>> iterator() {
            return new MapIterator<Entry<K, V>>(root == null ? null : root.first()) {
                public Entry<K, V> next() {
                    return stepForward();
                }
            };
        }

        @Override public boolean contains(Object o) {
            return o instanceof Entry && findByEntry((Entry<?, ?>) o) != null;
        }

        @Override public boolean remove(Object o) {
            if (!(o instanceof Entry)) {
                return false;
            }

            Node<K, V> node = findByEntry((Entry<?, ?>) o);
            if (node == null) {
                return false;
            }
            removeInternal(node);
            return true;
        }

        @Override public void clear() {
            TreeMap.this.clear();
        }
    }

    class KeySet extends AbstractSet<K> implements NavigableSet<K> {
        @Override public int size() {
            return size;
        }

        @Override public Iterator<K> iterator() {
            return new MapIterator<K>(root == null ? null : root.first()) {
                public K next() {
                    return stepForward().key;
                }
            };
        }

        public Iterator<K> descendingIterator() {
            return new MapIterator<K>(root == null ? null : root.last()) {
                public K next() {
                    return stepBackward().key;
                }
            };
        }

        @Override public boolean contains(Object o) {
            return containsKey(o);
        }

        @Override public boolean remove(Object key) {
            return removeInternalByKey(key) != null;
        }

        @Override public void clear() {
            TreeMap.this.clear();
        }

        public Comparator<? super K> comparator() {
            return TreeMap.this.comparator();
        }

        /*
         * Navigable methods.
         */

        public K first() {
            return firstKey();
        }

        public K last() {
            return lastKey();
        }

        public K lower(K key) {
            return lowerKey(key);
        }

        public K floor(K key) {
            return floorKey(key);
        }

        public K ceiling(K key) {
            return ceilingKey(key);
        }

        public K higher(K key) {
            return higherKey(key);
        }

        public K pollFirst() {
            Entry<K, V> entry = internalPollFirstEntry();
            return entry != null ? entry.getKey() : null;
        }

        public K pollLast() {
            Entry<K, V> entry = internalPollLastEntry();
            return entry != null ? entry.getKey() : null;
        }

        /*
         * View factory methods.
         */

        public NavigableSet<K> subSet(K from, boolean fromInclusive, K to, boolean toInclusive) {
            return TreeMap.this.subMap(from, fromInclusive, to, toInclusive).navigableKeySet();
        }

        public SortedSet<K> subSet(K fromInclusive, K toExclusive) {
            return TreeMap.this.subMap(fromInclusive, true, toExclusive, false).navigableKeySet();
        }

        public NavigableSet<K> headSet(K to, boolean inclusive) {
            return TreeMap.this.headMap(to, inclusive).navigableKeySet();
        }

        public SortedSet<K> headSet(K toExclusive) {
            return TreeMap.this.headMap(toExclusive, false).navigableKeySet();
        }

        public NavigableSet<K> tailSet(K from, boolean inclusive) {
            return TreeMap.this.tailMap(from, inclusive).navigableKeySet();
        }

        public SortedSet<K> tailSet(K fromInclusive) {
            return TreeMap.this.tailMap(fromInclusive, true).navigableKeySet();
        }

        public NavigableSet<K> descendingSet() {
            return new BoundedMap(false, null, NO_BOUND, null, NO_BOUND).navigableKeySet();
        }
    }

    /*
     * Bounded views implementations.
     */

    enum Bound {
        INCLUSIVE {
            @Override public String leftCap(Object from) {
                return "[" + from;
            }
            @Override public String rightCap(Object to) {
                return to + "]";
            }
        },
        EXCLUSIVE {
            @Override public String leftCap(Object from) {
                return "(" + from;
            }
            @Override public String rightCap(Object to) {
                return to + ")";
            }
        },
        NO_BOUND {
            @Override public String leftCap(Object from) {
                return ".";
            }
            @Override public String rightCap(Object to) {
                return ".";
            }
        };

        public abstract String leftCap(Object from);
        public abstract String rightCap(Object to);
    }

    /**
     * A map with optional limits on its range.
     */
    final class BoundedMap extends AbstractMap<K, V> implements NavigableMap<K, V>, Serializable {
        private final transient boolean ascending;
        private final transient K from;
        private final transient Bound fromBound;
        private final transient K to;
        private final transient Bound toBound;

        BoundedMap(boolean ascending, K from, Bound fromBound, K to, Bound toBound) {
            /*
             * Validate the bounds. In addition to checking that from <= to, we
             * verify that the comparator supports our bound objects.
             */
            if (fromBound != NO_BOUND && toBound != NO_BOUND) {
                if (comparator.compare(from, to) > 0) {
                    throw new IllegalArgumentException(from + " > " + to);
                }
            } else if (fromBound != NO_BOUND) {
                comparator.compare(from, from);
            } else if (toBound != NO_BOUND) {
                comparator.compare(to, to);
            }

            this.ascending = ascending;
            this.from = from;
            this.fromBound = fromBound;
            this.to = to;
            this.toBound = toBound;
        }

        @Override public int size() {
            return count(entrySet().iterator());
        }

        @Override public boolean isEmpty() {
            return endpoint(true) == null;
        }

        @Override public V get(Object key) {
            return isInBounds(key) ? TreeMap.this.get(key) : null;
        }

        @Override public boolean containsKey(Object key) {
            return isInBounds(key) && TreeMap.this.containsKey(key);
        }

        @Override public V put(K key, V value) {
            if (!isInBounds(key)) {
                throw outOfBounds(key, fromBound, toBound);
            }
            return putInternal(key, value);
        }

        @Override public V remove(Object key) {
            return isInBounds(key) ? TreeMap.this.remove(key) : null;
        }

        /**
         * Returns true if the key is in bounds.
         */
        @SuppressWarnings("unchecked") // this method throws ClassCastExceptions!
        private boolean isInBounds(Object key) {
            return isInBounds((K) key, fromBound, toBound);
        }

        /**
         * Returns true if the key is in bounds. Use this overload with
         * NO_BOUND to skip bounds checking on either end.
         */
        private boolean isInBounds(K key, Bound fromBound, Bound toBound) {
            if (fromBound == INCLUSIVE) {
                if (comparator.compare(key, from) < 0) {
                    return false; // less than from
                }
            } else if (fromBound == EXCLUSIVE) {
                if (comparator.compare(key, from) <= 0) {
                    return false; // less than or equal to from
                }
            }

            if (toBound == INCLUSIVE) {
                if (comparator.compare(key, to) > 0) {
                    return false; // greater than 'to'
                }
            } else if (toBound == EXCLUSIVE) {
                if (comparator.compare(key, to) >= 0) {
                    return false; // greater than or equal to 'to'
                }
            }

            return true;
        }

        /**
         * Returns the entry if it is in bounds, or null if it is out of bounds.
         */
        private Node<K, V> bound(Node<K, V> node, Bound fromBound, Bound toBound) {
            return node != null && isInBounds(node.getKey(), fromBound, toBound) ? node : null;
        }

        /*
         * Navigable methods.
         */

        public Entry<K, V> firstEntry() {
            return immutableCopy(endpoint(true));
        }

        public Entry<K, V> pollFirstEntry() {
            Node<K, V> result = endpoint(true);
            if (result != null) {
                removeInternal(result);
            }
            return immutableCopy(result);
        }

        public K firstKey() {
            Entry<K, V> entry = endpoint(true);
            if (entry == null) {
                throw new NoSuchElementException();
            }
            return entry.getKey();
        }

        public Entry<K, V> lastEntry() {
            return immutableCopy(endpoint(false));
        }

        public Entry<K, V> pollLastEntry() {
            Node<K, V> result = endpoint(false);
            if (result != null) {
                removeInternal(result);
            }
            return immutableCopy(result);
        }

        public K lastKey() {
            Entry<K, V> entry = endpoint(false);
            if (entry == null) {
                throw new NoSuchElementException();
            }
            return entry.getKey();
        }

        /**
         * @param first true for the first element, false for the last.
         */
        private Node<K, V> endpoint(boolean first) {
            Node<K, V> node;
            if (ascending == first) {
                switch (fromBound) {
                    case NO_BOUND:
                        node = root == null ? null : root.first();
                        break;
                    case INCLUSIVE:
                        node = find(from, CEILING);
                        break;
                    case EXCLUSIVE:
                        node = find(from, HIGHER);
                        break;
                    default:
                        throw new AssertionError();
                }
                return bound(node, NO_BOUND, toBound);
            } else {
                switch (toBound) {
                    case NO_BOUND:
                        node = root == null ? null : root.last();
                        break;
                    case INCLUSIVE:
                        node = find(to, FLOOR);
                        break;
                    case EXCLUSIVE:
                        node = find(to, LOWER);
                        break;
                    default:
                        throw new AssertionError();
                }
                return bound(node, fromBound, NO_BOUND);
            }
        }

        /**
         * Performs a find on the underlying tree after constraining it to the
         * bounds of this view. Examples:
         *
         *   bound is (A..C)
         *   findBounded(B, FLOOR) stays source.find(B, FLOOR)
         *
         *   bound is (A..C)
         *   findBounded(C, FLOOR) becomes source.find(C, LOWER)
         *
         *   bound is (A..C)
         *   findBounded(D, LOWER) becomes source.find(C, LOWER)
         *
         *   bound is (A..C]
         *   findBounded(D, FLOOR) becomes source.find(C, FLOOR)
         *
         *   bound is (A..C]
         *   findBounded(D, LOWER) becomes source.find(C, FLOOR)
         */
        private Entry<K, V> findBounded(K key, Relation relation) {
            relation = relation.forOrder(ascending);
            Bound fromBoundForCheck = fromBound;
            Bound toBoundForCheck = toBound;

            if (toBound != NO_BOUND && (relation == LOWER || relation == FLOOR)) {
                int comparison = comparator.compare(to, key);
                if (comparison <= 0) {
                    key = to;
                    if (toBound == EXCLUSIVE) {
                        relation = LOWER; // 'to' is too high
                    } else if (comparison < 0) {
                        relation = FLOOR; // we already went lower
                    }
                }
                toBoundForCheck = NO_BOUND; // we've already checked the upper bound
            }

            if (fromBound != NO_BOUND && (relation == CEILING || relation == HIGHER)) {
                int comparison = comparator.compare(from, key);
                if (comparison >= 0) {
                    key = from;
                    if (fromBound == EXCLUSIVE) {
                        relation = HIGHER; // 'from' is too low
                    } else if (comparison > 0) {
                        relation = CEILING; // we already went higher
                    }
                }
                fromBoundForCheck = NO_BOUND; // we've already checked the lower bound
            }

            return bound(find(key, relation), fromBoundForCheck, toBoundForCheck);
        }

        public Entry<K, V> lowerEntry(K key) {
            return immutableCopy(findBounded(key, LOWER));
        }

        public K lowerKey(K key) {
            Entry<K, V> entry = findBounded(key, LOWER);
            return entry != null ? entry.getKey() : null;
        }

        public Entry<K, V> floorEntry(K key) {
            return immutableCopy(findBounded(key, FLOOR));
        }

        public K floorKey(K key) {
            Entry<K, V> entry = findBounded(key, FLOOR);
            return entry != null ? entry.getKey() : null;
        }

        public Entry<K, V> ceilingEntry(K key) {
            return immutableCopy(findBounded(key, CEILING));
        }

        public K ceilingKey(K key) {
            Entry<K, V> entry = findBounded(key, CEILING);
            return entry != null ? entry.getKey() : null;
        }

        public Entry<K, V> higherEntry(K key) {
            return immutableCopy(findBounded(key, HIGHER));
        }

        public K higherKey(K key) {
            Entry<K, V> entry = findBounded(key, HIGHER);
            return entry != null ? entry.getKey() : null;
        }

        public Comparator<? super K> comparator() {
          if (ascending) {
            return TreeMap.this.comparator();
          } else {
            return Collections.reverseOrder(comparator);
          }
        }

        /*
         * View factory methods.
         */

        private transient BoundedEntrySet entrySet;
        private transient BoundedKeySet keySet;

        @Override public Set<Entry<K, V>> entrySet() {
            BoundedEntrySet result = entrySet;
            return result != null ? result : (entrySet = new BoundedEntrySet());
        }

        @Override public Set<K> keySet() {
            return navigableKeySet();
        }

        public NavigableSet<K> navigableKeySet() {
            BoundedKeySet result = keySet;
            return result != null ? result : (keySet = new BoundedKeySet());
        }

        public NavigableMap<K, V> descendingMap() {
            return new BoundedMap(!ascending, from, fromBound, to, toBound);
        }

        public NavigableSet<K> descendingKeySet() {
            return new BoundedMap(!ascending, from, fromBound, to, toBound).navigableKeySet();
        }

        public NavigableMap<K, V> subMap(K from, boolean fromInclusive, K to, boolean toInclusive) {
            Bound fromBound = fromInclusive ? INCLUSIVE : EXCLUSIVE;
            Bound toBound = toInclusive ? INCLUSIVE : EXCLUSIVE;
            return subMap(from, fromBound, to, toBound);
        }

        public NavigableMap<K, V> subMap(K fromInclusive, K toExclusive) {
            return subMap(fromInclusive, INCLUSIVE, toExclusive, EXCLUSIVE);
        }

        public NavigableMap<K, V> headMap(K to, boolean inclusive) {
            Bound toBound = inclusive ? INCLUSIVE : EXCLUSIVE;
            return subMap(null, NO_BOUND, to, toBound);
        }

        public NavigableMap<K, V> headMap(K toExclusive) {
            return subMap(null, NO_BOUND, toExclusive, EXCLUSIVE);
        }

        public NavigableMap<K, V> tailMap(K from, boolean inclusive) {
            Bound fromBound = inclusive ? INCLUSIVE : EXCLUSIVE;
            return subMap(from, fromBound, null, NO_BOUND);
        }

        public NavigableMap<K, V> tailMap(K fromInclusive) {
            return subMap(fromInclusive, INCLUSIVE, null, NO_BOUND);
        }

        private NavigableMap<K, V> subMap(K from, Bound fromBound, K to, Bound toBound) {
            if (!ascending) {
                K fromTmp = from;
                Bound fromBoundTmp = fromBound;
                from = to;
                fromBound = toBound;
                to = fromTmp;
                toBound = fromBoundTmp;
            }

            /*
             * If both the current and requested bounds are exclusive, the isInBounds check must be
             * inclusive. For example, to create (C..F) from (A..F), the bound 'F' is in bounds.
             */

            if (fromBound == NO_BOUND) {
                from = this.from;
                fromBound = this.fromBound;
            } else {
                Bound fromBoundToCheck = fromBound == this.fromBound ? INCLUSIVE : this.fromBound;
                if (!isInBounds(from, fromBoundToCheck, this.toBound)) {
                    throw outOfBounds(to, fromBoundToCheck, this.toBound);
                }
            }

            if (toBound == NO_BOUND) {
                to = this.to;
                toBound = this.toBound;
            } else {
                Bound toBoundToCheck = toBound == this.toBound ? INCLUSIVE : this.toBound;
                if (!isInBounds(to, this.fromBound, toBoundToCheck)) {
                    throw outOfBounds(to, this.fromBound, toBoundToCheck);
                }
            }

            return new BoundedMap(ascending, from, fromBound, to, toBound);
        }

        private IllegalArgumentException outOfBounds(Object value, Bound fromBound, Bound toBound) {
            return new IllegalArgumentException(value + " not in range "
                    + fromBound.leftCap(from) + ".." + toBound.rightCap(to));
        }

        /*
         * Bounded view implementations.
         */

        abstract class BoundedIterator<T> extends MapIterator<T> {
            protected BoundedIterator(Node<K, V> next) {
                super(next);
            }

            @Override protected Node<K, V> stepForward() {
                Node<K, V> result = super.stepForward();
                if (next != null && !isInBounds(next.key, NO_BOUND, toBound)) {
                    next = null;
                }
                return result;
            }

            @Override protected Node<K, V> stepBackward() {
                Node<K, V> result = super.stepBackward();
                if (next != null && !isInBounds(next.key, fromBound, NO_BOUND)) {
                    next = null;
                }
                return result;
            }
        }

        final class BoundedEntrySet extends AbstractSet<Entry<K, V>> {
            @Override public int size() {
                return BoundedMap.this.size();
            }

            @Override public boolean isEmpty() {
                return BoundedMap.this.isEmpty();
            }

            @Override public Iterator<Entry<K, V>> iterator() {
                return new BoundedIterator<Entry<K, V>>(endpoint(true)) {
                    public Entry<K, V> next() {
                        return ascending ? stepForward() : stepBackward();
                    }
                };
            }

            @Override public boolean contains(Object o) {
                if (!(o instanceof Entry)) {
                    return false;
                }
                Entry<?, ?> entry = (Entry<?, ?>) o;
                return isInBounds(entry.getKey()) && findByEntry(entry) != null;
            }

            @Override public boolean remove(Object o) {
                if (!(o instanceof Entry)) {
                    return false;
                }
                Entry<?, ?> entry = (Entry<?, ?>) o;
                return isInBounds(entry.getKey()) && TreeMap.this.entrySet().remove(entry);
            }
        }

        final class BoundedKeySet extends AbstractSet<K> implements NavigableSet<K> {
            @Override public int size() {
                return BoundedMap.this.size();
            }

            @Override public boolean isEmpty() {
                return BoundedMap.this.isEmpty();
            }

            @Override public Iterator<K> iterator() {
                return new BoundedIterator<K>(endpoint(true)) {
                    public K next() {
                        return (ascending ? stepForward() : stepBackward()).key;
                    }
                };
            }

            public Iterator<K> descendingIterator() {
                return new BoundedIterator<K>(endpoint(false)) {
                    public K next() {
                        return (ascending ? stepBackward() : stepForward()).key;
                    }
                };
            }

            @Override public boolean contains(Object key) {
                return isInBounds(key) && findByObject(key) != null;
            }

            @Override public boolean remove(Object key) {
                return isInBounds(key) && removeInternalByKey(key) != null;
            }

            /*
             * Navigable methods.
             */

            public K first() {
                return firstKey();
            }

            public K pollFirst() {
                Entry<K, ?> entry = pollFirstEntry();
                return entry != null ? entry.getKey() : null;
            }

            public K last() {
                return lastKey();
            }

            public K pollLast() {
                Entry<K, ?> entry = pollLastEntry();
                return entry != null ? entry.getKey() : null;
            }

            public K lower(K key) {
                return lowerKey(key);
            }

            public K floor(K key) {
                return floorKey(key);
            }

            public K ceiling(K key) {
                return ceilingKey(key);
            }

            public K higher(K key) {
                return higherKey(key);
            }

            public Comparator<? super K> comparator() {
                return BoundedMap.this.comparator();
            }

            /*
             * View factory methods.
             */

            public NavigableSet<K> subSet(K from, boolean fromInclusive, K to, boolean toInclusive) {
                return subMap(from, fromInclusive, to, toInclusive).navigableKeySet();
            }

            public SortedSet<K> subSet(K fromInclusive, K toExclusive) {
                return subMap(fromInclusive, toExclusive).navigableKeySet();
            }

            public NavigableSet<K> headSet(K to, boolean inclusive) {
                return headMap(to, inclusive).navigableKeySet();
            }

            public SortedSet<K> headSet(K toExclusive) {
                return headMap(toExclusive).navigableKeySet();
            }

            public NavigableSet<K> tailSet(K from, boolean inclusive) {
                return tailMap(from, inclusive).navigableKeySet();
            }

            public SortedSet<K> tailSet(K fromInclusive) {
                return tailMap(fromInclusive).navigableKeySet();
            }

            public NavigableSet<K> descendingSet() {
                return new BoundedMap(!ascending, from, fromBound, to, toBound).navigableKeySet();
            }
        }

        Object writeReplace() throws ObjectStreamException {
            return ascending
                    ? new AscendingSubMap<K, V>(TreeMap.this, from, fromBound, to, toBound)
                    : new DescendingSubMap<K, V>(TreeMap.this, from, fromBound, to, toBound);
        }
    }

    /**
     * Returns the number of elements in the iteration.
     */
    static int count(Iterator<?> iterator) {
        int count = 0;
        while (iterator.hasNext()) {
            iterator.next();
            count++;
        }
        return count;
    }

    /*
     * Serialization
     */

    private static final long serialVersionUID = 919286545866124006L;

    private void writeObject(ObjectOutputStream stream) throws IOException {
        stream.putFields().put("comparator", comparator != NATURAL_ORDER ? comparator : null);
        stream.writeFields();
        stream.writeInt(size);
        for (Map.Entry<K, V> entry : entrySet()) {
            stream.writeObject(entry.getKey());
            stream.writeObject(entry.getValue());
        }
    }

    @SuppressWarnings("unchecked") // we have to trust that keys are Ks and values are Vs
    private void readObject(ObjectInputStream stream) throws IOException, ClassNotFoundException {
        GetField fields = stream.readFields();
        comparator = (Comparator<? super K>) fields.get("comparator", null);
        if (comparator == null) {
            comparator = (Comparator<? super K>) NATURAL_ORDER;
        }
        int size = stream.readInt();
        for (int i = 0; i < size; i++) {
            putInternal((K) stream.readObject(), (V) stream.readObject());
        }
    }

    static abstract class NavigableSubMap<K, V> extends AbstractMap<K, V> implements Serializable {
        private static final long serialVersionUID = -2102997345730753016L;
        TreeMap<K, V> m;
        Object lo;
        Object hi;
        boolean fromStart;
        boolean toEnd;
        boolean loInclusive;
        boolean hiInclusive;

        NavigableSubMap(TreeMap<K, V> delegate, K from, Bound fromBound, K to, Bound toBound) {
            this.m = delegate;
            this.lo = from;
            this.hi = to;
            this.fromStart = fromBound == NO_BOUND;
            this.toEnd = toBound == NO_BOUND;
            this.loInclusive = fromBound == INCLUSIVE;
            this.hiInclusive = toBound == INCLUSIVE;
        }

        @Override public Set<Entry<K, V>> entrySet() {
            throw new UnsupportedOperationException();
        }

        @SuppressWarnings("unchecked") // we have to trust that the bounds are Ks
        protected Object readResolve() throws ObjectStreamException {
            Bound fromBound = fromStart ? NO_BOUND : (loInclusive ? INCLUSIVE : EXCLUSIVE);
            Bound toBound = toEnd ? NO_BOUND : (hiInclusive ? INCLUSIVE : EXCLUSIVE);
            boolean ascending = !(this instanceof DescendingSubMap);
            return m.new BoundedMap(ascending, (K) lo, fromBound, (K) hi, toBound);
        }
    }

    static class DescendingSubMap<K, V> extends NavigableSubMap<K, V> {
        private static final long serialVersionUID = 912986545866120460L;
        Comparator<K> reverseComparator;
        DescendingSubMap(TreeMap<K, V> delegate, K from, Bound fromBound, K to, Bound toBound) {
            super(delegate, from, fromBound, to, toBound);
        }
    }

    static class AscendingSubMap<K, V> extends NavigableSubMap<K, V> {
        private static final long serialVersionUID = 912986545866124060L;
        AscendingSubMap(TreeMap<K, V> delegate, K from, Bound fromBound, K to, Bound toBound) {
            super(delegate, from, fromBound, to, toBound);
        }
    }

    class SubMap extends AbstractMap<K, V> implements Serializable {
        private static final long serialVersionUID = -6520786458950516097L;
        Object fromKey;
        Object toKey;
        boolean fromStart;
        boolean toEnd;

        @Override public Set<Entry<K, V>> entrySet() {
            throw new UnsupportedOperationException();
        }

        @SuppressWarnings("unchecked") // we have to trust that the bounds are Ks
        protected Object readResolve() throws ObjectStreamException {
            Bound fromBound = fromStart ? NO_BOUND : INCLUSIVE;
            Bound toBound = toEnd ? NO_BOUND : EXCLUSIVE;
            return new BoundedMap(true, (K) fromKey, fromBound, (K) toKey, toBound);
        }
    }
}

		Implementations
		Hash Table	Resizable Array	Balanced Tree	Linked List	Hash Table + Linked List
Interfaces	Set	HashSet		TreeSet		LinkedHashSet
	List		ArrayList		LinkedList
	Deque		ArrayDeque		LinkedList
	Map	HashMap		TreeMap		LinkedHashMap

		是否有序	是否允许重复	是否线程同步
Collection		否	是
List	ArrayList	是	是	否
	Vector			是
	LinkedList			否
Set	HashSet	否	否	否
	TreeSet	是		否
Map	HashMap	否	<key, value>， key不允许重复	否
	TreeMap	是		否
	Hashtable	否		是

How to use:http://www.cnblogs.com/kiant71/archive/2008/09/04/1752079.html