Java容器Map接口

　　Map接口容器存放的是key-value对，由于Map是按key索引的，因此 key 是不可重复的，但 value 允许重复。 下面简单介绍一下Map接口的实现，包括HashMap，LinkedHashMap，WeakHashMap，Hashtable，IdentityHashMap和TreeMap。需要注意的是，Map接口并没有继承Collection接口！

 1、HashMap

　　HashMap 继承于AbstractMap，实现了Cloneable、java.io.Serializable接口，而AbstractMap实现了Map接口。HashMap 的实现不是同步的，这意味着它不是线程安全的。它的key、value都可以为null。此外，HashMap依赖key哈希值，所以其不是有序的，其底层使用数组（元素为链表，根据元素的hashcode确定数组下表）来存储数据。

public class HashMap<K,V>
    extends AbstractMap<K,V>
    implements Map<K,V>, Cloneable, Serializable
{

public abstract class AbstractMap<K,V> implements Map<K,V> {
    /**
     * Sole constructor.  (For invocation by subclass constructors, typically
     * implicit.)
     */
    protected AbstractMap() {
    }

public interface Map<K,V> {
    // Query Operations

    /**
     * Returns the number of key-value mappings in this map.  If the
     * map contains more than <tt>Integer.MAX_VALUE</tt> elements, returns
     * <tt>Integer.MAX_VALUE</tt>.
     *
     * @return the number of key-value mappings in this map
     */
    int size();

public class HashMap<K,V>
    extends AbstractMap<K,V>
    implements Map<K,V>, Cloneable, Serializable
{

    /**
     * The default initial capacity - MUST be a power of two.
     */
    static final int DEFAULT_INITIAL_CAPACITY = 16;

    /**
     * The maximum capacity, used if a higher value is implicitly specified
     * by either of the constructors with arguments.
     * MUST be a power of two <= 1<<30.
     */
    static final int MAXIMUM_CAPACITY = 1 << 30;

    /**
     * The load factor used when none specified in constructor.
     */
    static final float DEFAULT_LOAD_FACTOR = 0.75f;

    /**
     * The table, resized as necessary. Length MUST Always be a power of two.
     */
    transient Entry[] table;

    /**
     * The number of key-value mappings contained in this map.
     */
    transient int size;

    /**
     * The next size value at which to resize (capacity * load factor).
     * @serial
     */
    int threshold;

    /**
     * The load factor for the hash table.
     *
     * @serial
     */
    final float loadFactor;

    /**
     * The number of times this HashMap has been structurally modified
     * Structural modifications are those that change the number of mappings in
     * the HashMap or otherwise modify its internal structure (e.g.,
     * rehash).  This field is used to make iterators on Collection-views of
     * the HashMap fail-fast.  (See ConcurrentModificationException).
     */
    transient volatile int modCount;

    /**
     * Constructs an empty <tt>HashMap</tt> with the specified initial
     * capacity and load factor.
     *
     * @param  initialCapacity the initial capacity
     * @param  loadFactor      the load factor
     * @throws IllegalArgumentException if the initial capacity is negative
     *         or the load factor is nonpositive
     */
    public HashMap(int initialCapacity, float loadFactor) {
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal initial capacity: " +
                                               initialCapacity);
        if (initialCapacity > MAXIMUM_CAPACITY)
            initialCapacity = MAXIMUM_CAPACITY;
        if (loadFactor <= 0 || Float.isNaN(loadFactor))
            throw new IllegalArgumentException("Illegal load factor: " +
                                               loadFactor);

        // Find a power of 2 >= initialCapacity
        int capacity = 1;
        while (capacity < initialCapacity)
            capacity <<= 1;

        this.loadFactor = loadFactor;
        threshold = (int)(capacity * loadFactor);
        table = new Entry[capacity];
        init();
    }
 static class Entry<K,V> implements Map.Entry<K,V> {
        final K key;
        V value;
        Entry<K,V> next;
        final int hash;

        /**
         * Creates new entry.
         */
        Entry(int h, K k, V v, Entry<K,V> n) {
            value = v;
            next = n;
            key = k;
            hash = h;
        }

　　HashMap 的实例有两个参数影响其性能：“初始容量” 和 “加载因子”。容量是哈希表中桶的数量，初始容量只是哈希表在创建时的容量。加载因子是哈希表在其容量自动增加之前可以达到多满的一种尺度。当哈希表中的条目数超出了加载因子与当前容量的乘积时，则要对该哈希表进行resize操作（即重建内部数据结构），从而哈希表将具有大约两倍的桶数。通常，默认加载因子是 0.75, 这是在时间和空间成本上寻求一种折衷。

    public V put(K key, V value) {
        if (key == null)
            return putForNullKey(value);
        int hash = hash(key.hashCode());
        int i = indexFor(hash, table.length);
        for (Entry<K,V> e = table[i]; e != null; e = e.next) {
            Object k;
            if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
                V oldValue = e.value;
                e.value = value;
                e.recordAccess(this);
                return oldValue;
            }
        }

        modCount++;
        addEntry(hash, key, value, i);
        return null;
    }

    void addEntry(int hash, K key, V value, int bucketIndex) {
    Entry<K,V> e = table[bucketIndex];
        table[bucketIndex] = new Entry<K,V>(hash, key, value, e); // 在链表头加上此Entry
        if (size++ >= threshold)
            resize(2 * table.length);
    }

    void resize(int newCapacity) {
        Entry[] oldTable = table;
        int oldCapacity = oldTable.length;
        if (oldCapacity == MAXIMUM_CAPACITY) {
            threshold = Integer.MAX_VALUE;
            return;
        }

        Entry[] newTable = new Entry[newCapacity];
        transfer(newTable);
        table = newTable;
        threshold = (int)(newCapacity * loadFactor);
    }

2、LinkedHashMap

　　LinkedHashMap继承与HashMap，与HashMap相比LinkedHashMap维护的是一个具有双重链表的HashMap，LinkedHashMap支持排序，输出时其元素是有顺序的，先插入的先遍历到（对链表的遍历都是使用这个双向链表），而HashMap输出时是随机的。如果Map映射比较复杂而又要求高效率的话，最好使用LinkedHashMap。同样，LinkedHashMap也是非线程安全的。

　　友情提示一下，可以利用LinkedHashMap很方便的实现LRU算法，主要是重写boolean removeEldestEntry(Map.Entry<K,V> eldest)方法。如果应该从映射移除最旧的条目，则返回 true；如果应该保留，则返回 false。

public class LinkedHashMap<K,V>
    extends HashMap<K,V>
    implements Map<K,V>
{

    private static final long serialVersionUID = 3801124242820219131L;

    /**
     * The head of the doubly linked list.
     */
    private transient Entry<K,V> header; // 较HashMap多维护的一条双向链表

    /**
     * This override alters behavior of superclass put method. It causes newly
     * allocated entry to get inserted at the end of the linked list and
     * removes the eldest entry if appropriate.
     */
    void addEntry(int hash, K key, V value, int bucketIndex) {
        createEntry(hash, key, value, bucketIndex);

        // Remove eldest entry if instructed, else grow capacity if appropriate
        Entry<K,V> eldest = header.after;
        if (removeEldestEntry(eldest)) {
            removeEntryForKey(eldest.key);
        } else {
            if (size >= threshold)
                resize(2 * table.length);
        }
    }

3、WeakHashMap

　　WeakHashMap继承了AbstractMap而且实现了Map接口。WeakHashMap的每一个key对象保存了实际对象的弱引用，当系统回收了该key所对应的实际对象之后，WeakHashMap会自动删除该key对应的key-value对（http://zhang-xzhi-xjtu.iteye.com/blog/413159）。虽然Java有 垃圾回收机制，但是只要是自己管理的内存，就应该警惕内存泄露的问题，例如的对象池、缓存中的过期对象都有可能引发内存泄露的问题，其中我们可以用 WeakHashMap来作为缓存的容器可以有效解决这一问题。

　　WeakHashMap维护了一个ReferenceQueue，保存了所有存在引用的Key对象：

public class WeakHashMap<K,V>
    extends AbstractMap<K,V>
    implements Map<K,V> {

    /**
     * The default initial capacity -- MUST be a power of two.
     */
    private static final int DEFAULT_INITIAL_CAPACITY = 16;

    /**
     * The maximum capacity, used if a higher value is implicitly specified
     * by either of the constructors with arguments.
     * MUST be a power of two <= 1<<30.
     */
    private static final int MAXIMUM_CAPACITY = 1 << 30;

    /**
     * The load fast used when none specified in constructor.
     */
    private static final float DEFAULT_LOAD_FACTOR = 0.75f;

    /**
     * The table, resized as necessary. Length MUST Always be a power of two.
     */
    private Entry[] table;

    /**
     * The number of key-value mappings contained in this weak hash map.
     */
    private int size;

    /**
     * The next size value at which to resize (capacity * load factor).
     */
    private int threshold;

    /**
     * The load factor for the hash table.
     */
    private final float loadFactor;

    /**
     * Reference queue for cleared WeakEntries
     */
    private final ReferenceQueue<K> queue = new ReferenceQueue<K>(); // 这个是重点，引用队列

 /**
     * The entries in this hash table extend WeakReference, using its main ref
     * field as the key.
     */
    private static class Entry<K,V> extends WeakReference<K> implements Map.Entry<K,V> {
        private V value;
        private final int hash;
        private Entry<K,V> next;

        /**
         * Creates new entry.
         */
        Entry(K key, V value,
          ReferenceQueue<K> queue,
              int hash, Entry<K,V> next) {
            super(key, queue);
            this.value = value;
            this.hash  = hash;
            this.next  = next;
        }

　　同时， WeakHashMap中有一个私有的expungeStaleEntries（）方法，会在大部分共有方法中被调用，而且这个方法会将ReferenceQueue中所有失效的引用从Map中去除，从而实现了weak的效果。

  /**
     * Expunges stale entries from the table.
     */
    private void expungeStaleEntries() {
    Entry<K,V> e;
        while ( (e = (Entry<K,V>) queue.poll()) != null) {
            int h = e.hash;
            int i = indexFor(h, table.length);

            Entry<K,V> prev = table[i];
            Entry<K,V> p = prev;
            while (p != null) {
                Entry<K,V> next = p.next;
                if (p == e) {
                    if (prev == e)
                        table[i] = next;
                    else
                        prev.next = next;
                    e.next = null;  // Help GC
                    e.value = null; //  "   "
                    size--;
                    break;
                }
                prev = p;
                p = next;
            }
        }
    }

4、HashTable

HashTable是比较久远以前的类了，其继承了Dictionary类，实现了Map接口。

public class Hashtable<K,V>
    extends Dictionary<K,V>
    implements Map<K,V>, Cloneable, java.io.Serializable {

    /**
     * The hash table data.
     */
    private transient Entry[] table;

    /**
     * The total number of entries in the hash table.
     */
    private transient int count;

    /**
     * The table is rehashed when its size exceeds this threshold.  (The
     * value of this field is (int)(capacity * loadFactor).)
     *
     * @serial
     */
    private int threshold;

    /**
     * The load factor for the hashtable.
     *
     * @serial
     */
    private float loadFactor;

    /**
     * The number of times this Hashtable has been structurally modified
     * Structural modifications are those that change the number of entries in
     * the Hashtable or otherwise modify its internal structure (e.g.,
     * rehash).  This field is used to make iterators on Collection-views of
     * the Hashtable fail-fast.  (See ConcurrentModificationException).
     */
    private transient int modCount = 0;

    /** use serialVersionUID from JDK 1.0.2 for interoperability */
    private static final long serialVersionUID = 1421746759512286392L;

    /**
     * Constructs a new, empty hashtable with the specified initial
     * capacity and the specified load factor.
     *
     * @param      initialCapacity   the initial capacity of the hashtable.
     * @param      loadFactor        the load factor of the hashtable.
     * @exception  IllegalArgumentException  if the initial capacity is less
     *             than zero, or if the load factor is nonpositive.
     */
    public Hashtable(int initialCapacity, float loadFactor) {
    if (initialCapacity < 0)
        throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        if (loadFactor <= 0 || Float.isNaN(loadFactor))
            throw new IllegalArgumentException("Illegal Load: "+loadFactor);

        if (initialCapacity==0)
            initialCapacity = 1;
    this.loadFactor = loadFactor;
    table = new Entry[initialCapacity];
    threshold = (int)(initialCapacity * loadFactor);
    }

　　HashTable是线程安全的，其很多方法的实现都是用了synchronized关键字：

    /**
     * Returns the number of keys in this hashtable.
     *
     * @return  the number of keys in this hashtable.
     */
    public synchronized int size() {
    return count;
    }

　　另外，HashTable在扩容的时候会rehash（HashMap已经用resize方法不会去rehash从而优化了这个的效率问题），这个是非常耗时的操作。同时，HashTable不允许存入null的key或者value，否则会抛出NullPointerException

 public synchronized V put(K key, V value) {
    // Make sure the value is not null
    if (value == null) {
        throw new NullPointerException();
    }

    // Makes sure the key is not already in the hashtable.
    Entry tab[] = table;
    int hash = key.hashCode();
    int index = (hash & 0x7FFFFFFF) % tab.length;
    for (Entry<K,V> e = tab[index] ; e != null ; e = e.next) {
        if ((e.hash == hash) && e.key.equals(key)) {
        V old = e.value;
        e.value = value;
        return old;
        }
    }

    modCount++;
    if (count >= threshold) {
        // Rehash the table if the threshold is exceeded
        rehash(); ///////////////////////////////////////////////////////////////////////////////////////

            tab = table;
            index = (hash & 0x7FFFFFFF) % tab.length;
    }

5、IdentityHashMap

　　IdentityHashMap类利用哈希表实现 Map 接口，与HashMap无太大区别。下面描述了IdentityHashMap和HashMap比较不一样的地方。

　　比较键（和值）时使用引用相等性代替对象相等性，换句话说，在 IdentityHashMap 中，当且仅当 (k1==k2) 时，才认为两个键 k1 和 k2 相等（在正常 Map 实现中，当且仅当满足(k1==null ? k2==null : e1.equals(e2))才认为两个键 k1 和 k2 相等）。

public V put(K key, V value) {
        Object k = maskNull(key);
        Object[] tab = table;
        int len = tab.length;
        int i = hash(k, len);

        Object item;
        while ( (item = tab[i]) != null) {
            if (item == k) {
		V oldValue = (V) tab[i + 1];
                tab[i + 1] = value;
                return oldValue;
            }
            i = nextKeyIndex(i, len);
        }

        modCount++;
        tab[i] = k;
        tab[i + 1] = value;
        if (++size >= threshold)
            resize(len); // len == 2 * current capacity.
        return null;
    }

　　默认的价值加载因子为2/3,在重新哈希后，加载因子变为1/3.当哈希表中的条目数超出了加载因子与当前容量的乘积时，通过调用 reszie 方法将容量翻倍,重新进行哈希。增加桶数，重新哈希，可能相当昂贵。

 private void resize(int newCapacity) {
        // assert (newCapacity & -newCapacity) == newCapacity; // power of 2
        int newLength = newCapacity * 2;

    Object[] oldTable = table;
        int oldLength = oldTable.length;
        if (oldLength == 2*MAXIMUM_CAPACITY) { // can't expand any further
            if (threshold == MAXIMUM_CAPACITY-1)
                throw new IllegalStateException("Capacity exhausted.");
            threshold = MAXIMUM_CAPACITY-1;  // Gigantic map!
            return;
        }
        if (oldLength >= newLength)
            return;

    Object[] newTable = new Object[newLength];
        threshold = newLength / 3;

        for (int j = 0; j < oldLength; j += 2) {
            Object key = oldTable[j];
            if (key != null) {
                Object value = oldTable[j+1];
                oldTable[j] = null;
                oldTable[j+1] = null;
                int i = hash(key, newLength); ////////////////////这里重哈希了/////////////////////////
                while (newTable[i] != null)
                    i = nextKeyIndex(i, newLength);
                newTable[i] = key;
                newTable[i + 1] = value;
            }
        }
        table = newTable;
    }

6、TreeMap

　　TreeMap 的实现就是红黑树数据结构，也就说是一棵自平衡的排序二叉树，这样就可以保证当需要快速检索指定节点。红黑树是一种自平衡排序二叉树，树中每个节点的值，都大于或等于在它的左子树中的所有节点的值，并且小于或等于在它的右子树中的所有节点的值，这确保红黑树运行时可以快速地在树中查找和定位的所需节点。 对于 TreeMap 而言，由于它底层采用一棵“红黑树”来保存集合中的 Entry，这意味这 TreeMap 添加元素、取出元素的性能都比 HashMap 低：当 TreeMap 添加元素时，需要通过循环找到新增 Entry 的插入位置，因此比较耗性能；当从 TreeMap 中取出元素时，需要通过循环才能找到合适的 Entry，也比较耗性能。但 TreeMap比 HashMap 的优势在于：TreeMap 中的所有 Entry 总是按 key 根据指定排序规则保持有序状态，TreeSet 中所有元素总是根据指定排序规则保持有序状态。

public class TreeMap<K,V>
    extends AbstractMap<K,V>
    implements NavigableMap<K,V>, Cloneable, java.io.Serializable
{
    /**
     * The comparator used to maintain order in this tree map, or
     * null if it uses the natural ordering of its keys.
     *
     * @serial
     */
    private final Comparator<? super K> comparator;

    private transient Entry<K,V> root = null;

    /**
     * The number of entries in the tree
     */
    private transient int size = 0;

    /**
     * The number of structural modifications to the tree.
     */
    private transient int modCount = 0;

    /**
     * Constructs a new, empty tree map, using the natural ordering of its
     * keys.  All keys inserted into the map must implement the {@link
     * Comparable} interface.  Furthermore, all such keys must be
     * <i>mutually comparable</i>: <tt>k1.compareTo(k2)</tt> must not throw
     * a <tt>ClassCastException</tt> for any keys <tt>k1</tt> and
     * <tt>k2</tt> in the map.  If the user attempts to put a key into the
     * map that violates this constraint (for example, the user attempts to
     * put a string key into a map whose keys are integers), the
     * <tt>put(Object key, Object value)</tt> call will throw a
     * <tt>ClassCastException</tt>.
     */
    public TreeMap() {
        comparator = null;
    }

    /**
     * Constructs a new, empty tree map, ordered according to the given
     * comparator.  All keys inserted into the map must be <i>mutually
     * comparable</i> by the given comparator: <tt>comparator.compare(k1,
     * k2)</tt> must not throw a <tt>ClassCastException</tt> for any keys
     * <tt>k1</tt> and <tt>k2</tt> in the map.  If the user attempts to put
     * a key into the map that violates this constraint, the <tt>put(Object
     * key, Object value)</tt> call will throw a
     * <tt>ClassCastException</tt>.
     *
     * @param comparator the comparator that will be used to order this map.
     *        If <tt>null</tt>, the {@linkplain Comparable natural
     *        ordering} of the keys will be used.
     */
    public TreeMap(Comparator<? super K> comparator) {
        this.comparator = comparator;
    }

 static final class Entry<K,V> implements Map.Entry<K,V> {
    K key;
        V value;
        Entry<K,V> left = null;
        Entry<K,V> right = null;
        Entry<K,V> parent;
        boolean color = BLACK;

        /**
         * Make a new cell with given key, value, and parent, and with
         * <tt>null</tt> child links, and BLACK color.
         */
        Entry(K key, V value, Entry<K,V> parent) {
            this.key = key;
            this.value = value;
            this.parent = parent;
        }

　　同样是支持有序状态，LinkedHashMap保存了记录的插入顺序，先插入的先遍历到，而TreeMap默认是按升序排，也可以指定排序的比较器，遍历的时候按升序遍历。这个TreeMap的实现比较复杂，有需要研究的朋友可以自行查看其源码。

　　最后，我们来讨论一下hashcode在Map中的重要性。从上面我们了解到Map主要是依赖Hash码来查找元素链表的。根据某个计算公式，使用hashcode找到Entry所在的段，最后遍历对应段的链表结构再次去对比hashcode以及key值，最后找到value。当key对象的hash方法设计不当时，可能导致单个链表数据量过大，最后线性查找性能下降，导致整个map的查找性能下降。