Collection是一个接口,它主要的两个分支是List和Set。如下图所示:
List和Set都是接口,它们继承与Collection。List是有序的队列,可以用重复的元素;而Set是数学概念中的集合,不能有重复的元素。List和Set都有它们各自的实现类。
为了方便,我们抽象出AbstractCollection类来让其他类继承,该类实现类Collection中的绝大部分方法。AbstractList和AbstractSet都继承与AbstractCollection,具体的List实现类继承与AbstractList,而Set的实现类则继承与AbstractSet。
另外,Collection中有个iterator()方法,它的作用是返回一个Iterator接口。通常,我们通过Iterator迭代器来遍历集合。ListIterator是List接口所特有的,在List接口中,通过ListIterator()返回一个ListIterator对象。
我们首先来阅读下这些 接口和抽象类以及他们的实现类中都有哪些方法:
1. Collection
Collection的定义如下:
public interface Collection<E> extends Iterable<E> {}
从它的定义中可以看出,Collection是一个接口。它是一个高度抽象出来的集合,包含了集合的基本操作:添加、删除、清空、遍历、是否为空、获取大小等。
Collection接口的所有子类(直接子类和简介子类)都必须实现2种构造函数:不带参数的构造函数和参数为Collection的构造函数。带参数的构造函数可以用来转换Collection的类型。下面是Collection接口中定义的API:
// Collection的API abstract boolean add(E object) abstract boolean addAll(Collection<? extends E> collection) abstract void clear() abstract boolean contains(Object object) abstract boolean containsAll(Collection<?> collection) abstract boolean equals(Object object) abstract int hashCode() abstract boolean isEmpty() abstract Iterator<E> iterator() abstract boolean remove(Object object) abstract boolean removeAll(Collection<?> collection) abstract boolean retainAll(Collection<?> collection) abstract int size() abstract <T> T[] toArray(T[] array) abstract Object[] toArray()
2. List
List的定义如下:
public interface List<E> extends Collection<E> {}
从List定义中可以看出,它继承与Collection接口,即List是集合的一种。List是有序的队列,List中的每一个元素都有一个索引,第一个元素的索引值为0,往后的元素的索引值依次+1.,List中允许有重复的元素。
List继承Collection自然包含了Collection的所有接口,由于List是有序队列,所以它也有自己额外的API接口。API如下:
// Collection的API abstract boolean add(E object) abstract boolean addAll(Collection<? extends E> collection) abstract void clear() abstract boolean contains(Object object) abstract boolean containsAll(Collection<?> collection) abstract boolean equals(Object object) abstract int hashCode() abstract boolean isEmpty() abstract Iterator<E> iterator() abstract boolean remove(Object object) abstract boolean removeAll(Collection<?> collection) abstract boolean retainAll(Collection<?> collection) abstract int size() abstract <T> T[] toArray(T[] array) abstract Object[] toArray() // 相比与Collection,List新增的API: abstract void add(int location, E object) //在指定位置添加元素 abstract boolean addAll(int location, Collection<? extends E> collection) //在指定位置添加其他集合中的元素 abstract E get(int location) //获取指定位置的元素 abstract int indexOf(Object object) //获得指定元素的索引 abstract int lastIndexOf(Object object) //从右边的索引 abstract ListIterator<E> listIterator(int location) //获得iterator abstract ListIterator<E> listIterator() abstract E remove(int location) //删除指定位置的元素 abstract E set(int location, E object) //修改指定位置的元素 abstract List<E> subList(int start, int end) //获取子list
3. Set
Set的定义如下:
public interface Set<E> extends Collection<E> {}
Set也继承与Collection接口,且里面不能有重复元素。关于API,Set与Collection的API完全一样,不在赘述。
4. AbstractCollection
public abstract class AbstractCollection<E> implements Collection<E> {}
AbstractCollection是一个抽象类,它实现了Collection中除了iterator()和size()之外的所有方法。AbstractCollection的主要作用是方便其他类实现Collection.,比如ArrayList、LinkedList等。它们想要实现Collection接口,通过集成AbstractCollection就已经实现大部分方法了,再实现一下iterator()和size()即可。
下面看一下AbstractCollection实现的部分方法的源码:
public abstract class AbstractCollection<E> implements Collection<E> { protected AbstractCollection() { } public abstract Iterator<E> iterator();//iterator()方法没有实现 public abstract int size(); //size()方法也没有实现 public boolean isEmpty() { //检测集合是否为空 return size() == 0; } /*检查集合中是否包含特定对象*/ public boolean contains(Object o) { Iterator<E> it = iterator(); if (o==null) { while (it.hasNext()) //从这里可以看出,任何非空集合都包含null if (it.next()==null) return true; } else { while (it.hasNext()) if (o.equals(it.next())) return true; } return false; } /*将集合转变成数组*/ public Object[] toArray() { // Estimate size of array; be prepared to see more or fewer elements Object[] r = new Object[size()]; //创建与集合大小相同的数组 Iterator<E> it = iterator(); for (int i = 0; i < r.length; i++) { if (! it.hasNext()) // fewer elements than expected //Arrays.copy(**,**)的第二个参数是待copy的长度,如果这个长度大于r,则保留r的长度 return Arrays.copyOf(r, i); r[i] = it.next(); } return it.hasNext() ? finishToArray(r, it) : r; } public <T> T[] toArray(T[] a) { // Estimate size of array; be prepared to see more or fewer elements int size = size(); T[] r = a.length >= size ? a : (T[])java.lang.reflect.Array .newInstance(a.getClass().getComponentType(), size); Iterator<E> it = iterator(); for (int i = 0; i < r.length; i++) { if (! it.hasNext()) { // fewer elements than expected if (a == r) { r[i] = null; // null-terminate } else if (a.length < i) { return Arrays.copyOf(r, i); } else { System.arraycopy(r, 0, a, 0, i); if (a.length > i) { a[i] = null; } } return a; } r[i] = (T)it.next(); } // more elements than expected return it.hasNext() ? finishToArray(r, it) : r; } private static <T> T[] finishToArray(T[] r, Iterator<?> it) { int i = r.length; while (it.hasNext()) { int cap = r.length; if (i == cap) { int newCap = cap + (cap >> 1) + 1; // overflow-conscious code if (newCap - MAX_ARRAY_SIZE > 0) newCap = hugeCapacity(cap + 1); r = Arrays.copyOf(r, newCap); } r[i++] = (T)it.next(); } // trim if overallocated return (i == r.length) ? r : Arrays.copyOf(r, i); } private static int hugeCapacity(int minCapacity) { if (minCapacity < 0) // overflow throw new OutOfMemoryError ("Required array size too large"); return (minCapacity > MAX_ARRAY_SIZE) ? Integer.MAX_VALUE : MAX_ARRAY_SIZE; } // 删除对象o public boolean remove(Object o) { Iterator<E> it = iterator(); if (o==null) { while (it.hasNext()) { if (it.next()==null) { it.remove(); return true; } } } else { while (it.hasNext()) { if (o.equals(it.next())) { it.remove(); return true; } } } return false; } <pre name="code" class="java"> // 判断是否包含集合c中所有元素 public boolean containsAll(Collection<?> c) { for (Object e : c) if (!contains(e)) return false; return true; } //添加集合c中所有元素 public boolean addAll(Collection<? extends E> c) { boolean modified = false; for (E e : c) if (add(e)) modified = true; return modified; } //删除集合c中所有元素(如果存在的话) public boolean removeAll(Collection<?> c) { boolean modified = false; Iterator<?> it = iterator(); while (it.hasNext()) { if (c.contains(it.next())) { it.remove(); modified = true; } } return modified; } //清空 public void clear() { Iterator<E> it = iterator(); while (it.hasNext()) { it.next(); it.remove(); } } //将集合元素显示成[String] public String toString() { Iterator<E> it = iterator(); if (! it.hasNext()) return "[]"; StringBuilder sb = new StringBuilder(); sb.append('['); for (;;) { E e = it.next(); sb.append(e == this ? "(this Collection)" : e); if (! it.hasNext()) return sb.append(']').toString(); sb.append(',').append(' '); } } }
5. AbstractList
AbstractList的定义如下:
public abstract class AbstractList<E> extends AbstractCollection<E> implements List<E> {}
从定义中可以看出,AbstractList是一个继承AbstractCollection,并且实现了List接口的抽象类。它实现了List中除了size()、get(int location)之外的方法。
AbstractList的主要作用:它实现了List接口中的大部分函数,从而方便其它类继承List。另外,和AbstractCollection相比,AbstractList抽象类中,实现了iterator()方法。
AbstractList抽象类的源码如下:
![](https://images.cnblogs.com/OutliningIndicators/ContractedBlock.gif)
public abstract class AbstractList<E> extends AbstractCollection<E> implements List<E> { protected AbstractList() { } public boolean add(E e) { add(size(), e); return true; } abstract public E get(int index); public E set(int index, E element) { throw new UnsupportedOperationException(); } public void add(int index, E element) { throw new UnsupportedOperationException(); } public E remove(int index) { throw new UnsupportedOperationException(); } /***************************** Search Operations**********************************/ public int indexOf(Object o) { //搜索对象o的索引 ListIterator<E> it = listIterator(); if (o==null) { while (it.hasNext()) if (it.next()==null) //执行it.next(),会先返回it指向位置的值,然后it会移到下一个位置 return it.previousIndex(); //所以要返回it.previousIndex(); 关于it几个方法的源码在下面 } else { while (it.hasNext()) if (o.equals(it.next())) return it.previousIndex(); } return -1; } public int lastIndexOf(Object o) { ListIterator<E> it = listIterator(size()); if (o==null) { while (it.hasPrevious()) if (it.previous()==null) return it.nextIndex(); } else { while (it.hasPrevious()) if (o.equals(it.previous())) return it.nextIndex(); } return -1; } /**********************************************************************************/ /****************************** Bulk Operations ***********************************/ public void clear() { removeRange(0, size()); } public boolean addAll(int index, Collection<? extends E> c) { rangeCheckForAdd(index); boolean modified = false; for (E e : c) { add(index++, e); modified = true; } return modified; } protected void removeRange(int fromIndex, int toIndex) { ListIterator<E> it = listIterator(fromIndex); for (int i=0, n=toIndex-fromIndex; i<n; i++) { it.next(); it.remove(); } } /**********************************************************************************/ /********************************* Iterators **************************************/ public Iterator<E> iterator() { return new Itr(); } public ListIterator<E> listIterator() { return listIterator(0); //返回的iterator索引从0开始 } public ListIterator<E> listIterator(final int index) { rangeCheckForAdd(index); //首先检查index范围是否正确 return new ListItr(index); //ListItr继承与Itr且实现了ListIterator接口,Itr实现了Iterator接口,往下看 } private class Itr implements Iterator<E> { int cursor = 0; //元素的索引,当调用next()方法时,返回当前索引的值 int lastRet = -1; //lastRet也是元素的索引,但如果删掉此元素,该值置为-1 /* *迭代器都有个modCount值,在使用迭代器的时候,如果使用remove,add等方法的时候都会修改modCount, *在迭代的时候需要保持单线程的唯一操作,如果期间进行了插入或者删除,modCount就会被修改,迭代器就会检测到被并发修改,从而出现运行时异常。 *举个简单的例子,现在某个线程正在遍历一个List,另一个线程对List中的某个值做了删除,那原来的线程用原来的迭代器当然无法正常遍历了 */ int expectedModCount = modCount; public boolean hasNext() { return cursor != size(); //当索引值和元素个数相同时表示没有下一个元素了,索引是从0到size-1 } public E next() { checkForComodification(); //检查modCount是否改变 try { int i = cursor; //next()方法主要做了两件事: E next = get(i); lastRet = i; cursor = i + 1; //1.将索引指向了下一个位置 return next; //2. 返回当前索引的值 } catch (IndexOutOfBoundsException e) { checkForComodification(); throw new NoSuchElementException(); } } public void remove() { if (lastRet < 0) //lastRet<0表示已经不存在了 throw new IllegalStateException(); checkForComodification(); try { AbstractList.this.remove(lastRet); if (lastRet < cursor) cursor--; //原位置的索引值减小了1,但是实际位置没变 lastRet = -1; //置为-1表示已删除 expectedModCount = modCount; } catch (IndexOutOfBoundsException e) { throw new ConcurrentModificationException(); } } final void checkForComodification() { if (modCount != expectedModCount) throw new ConcurrentModificationException(); } } private class ListItr extends Itr implements ListIterator<E> { ListItr(int index) { cursor = index; } public boolean hasPrevious() { return cursor != 0; } public E previous() { checkForComodification(); try { int i = cursor - 1; //previous()方法中也做了两件事: E previous = get(i); //1. 将索引向前移动一位 lastRet = cursor = i; //2. 返回索引处的值 return previous; } catch (IndexOutOfBoundsException e) { checkForComodification(); throw new NoSuchElementException(); } } public int nextIndex() { //iterator中的index本来就是下一个位置,在next()方法中可以看出 return cursor; } public int previousIndex() { return cursor-1; } public void set(E e) { //修改当前位置的元素 if (lastRet < 0) throw new IllegalStateException(); checkForComodification(); try { AbstractList.this.set(lastRet, e); expectedModCount = modCount; } catch (IndexOutOfBoundsException ex) { throw new ConcurrentModificationException(); } } public void add(E e) { //在当前位置添加元素 checkForComodification(); try { int i = cursor; AbstractList.this.add(i, e); lastRet = -1; cursor = i + 1; expectedModCount = modCount; } catch (IndexOutOfBoundsException ex) { throw new ConcurrentModificationException(); } } } /**********************************************************************************/ //获得子List,详细源码往下看SubList类 public List<E> subList(int fromIndex, int toIndex) { return (this instanceof RandomAccess ? new RandomAccessSubList<>(this, fromIndex, toIndex) : new SubList<>(this, fromIndex, toIndex)); } /*************************** Comparison and hashing *******************************/ public boolean equals(Object o) { if (o == this) return true; if (!(o instanceof List)) return false; ListIterator<E> e1 = listIterator(); ListIterator e2 = ((List) o).listIterator(); while (e1.hasNext() && e2.hasNext()) { E o1 = e1.next(); Object o2 = e2.next(); if (!(o1==null ? o2==null : o1.equals(o2))) return false; } return !(e1.hasNext() || e2.hasNext()); } public int hashCode() { //hashcode int hashCode = 1; for (E e : this) hashCode = 31*hashCode + (e==null ? 0 : e.hashCode()); return hashCode; } /**********************************************************************************/ protected transient int modCount = 0; private void rangeCheckForAdd(int index) { if (index < 0 || index > size()) throw new IndexOutOfBoundsException(outOfBoundsMsg(index)); } private String outOfBoundsMsg(int index) { return "Index: "+index+", Size: "+size(); } } class SubList<E> extends AbstractList<E> { private final AbstractList<E> l; private final int offset; private int size; /* 从SubList源码可以看出,当需要获得一个子List时,底层并不是真正的返回一个子List,还是原来的List,只不过 * 在操作的时候,索引全部限定在用户所需要的子List部分而已 */ SubList(AbstractList<E> list, int fromIndex, int toIndex) { if (fromIndex < 0) throw new IndexOutOfBoundsException("fromIndex = " + fromIndex); if (toIndex > list.size()) throw new IndexOutOfBoundsException("toIndex = " + toIndex); if (fromIndex > toIndex) throw new IllegalArgumentException("fromIndex(" + fromIndex + ") > toIndex(" + toIndex + ")"); l = list; //原封不动的将原来的list赋给l offset = fromIndex; //偏移量,用在操作新的子List中 size = toIndex - fromIndex; //子List的大小,所以子List中不包括toIndex处的值,即子List中包括左边不包括右边 this.modCount = l.modCount; } //注意下面所有的操作都在索引上加上偏移量offset,相当于在原来List的副本上操作子List public E set(int index, E element) { rangeCheck(index); checkForComodification(); return l.set(index+offset, element); } public E get(int index) { rangeCheck(index); checkForComodification(); return l.get(index+offset); } public int size() { checkForComodification(); return size; } public void add(int index, E element) { rangeCheckForAdd(index); checkForComodification(); l.add(index+offset, element); this.modCount = l.modCount; size++; } public E remove(int index) { rangeCheck(index); checkForComodification(); E result = l.remove(index+offset); this.modCount = l.modCount; size--; return result; } protected void removeRange(int fromIndex, int toIndex) { checkForComodification(); l.removeRange(fromIndex+offset, toIndex+offset); this.modCount = l.modCount; size -= (toIndex-fromIndex); } public boolean addAll(Collection<? extends E> c) { return addAll(size, c); } public boolean addAll(int index, Collection<? extends E> c) { rangeCheckForAdd(index); int cSize = c.size(); if (cSize==0) return false; checkForComodification(); l.addAll(offset+index, c); this.modCount = l.modCount; size += cSize; return true; } public Iterator<E> iterator() { return listIterator(); } public ListIterator<E> listIterator(final int index) { checkForComodification(); rangeCheckForAdd(index); return new ListIterator<E>() { private final ListIterator<E> i = l.listIterator(index+offset); //相当子List的索引0 public boolean hasNext() { return nextIndex() < size; } public E next() { if (hasNext()) return i.next(); else throw new NoSuchElementException(); } public boolean hasPrevious() { return previousIndex() >= 0; } public E previous() { if (hasPrevious()) return i.previous(); else throw new NoSuchElementException(); } public int nextIndex() { return i.nextIndex() - offset; } public int previousIndex() { return i.previousIndex() - offset; } public void remove() { i.remove(); SubList.this.modCount = l.modCount; size--; } public void set(E e) { i.set(e); } public void add(E e) { i.add(e); SubList.this.modCount = l.modCount; size++; } }; } public List<E> subList(int fromIndex, int toIndex) { return new SubList<>(this, fromIndex, toIndex); } private void rangeCheck(int index) { if (index < 0 || index >= size) throw new IndexOutOfBoundsException(outOfBoundsMsg(index)); } private void rangeCheckForAdd(int index) { if (index < 0 || index > size) throw new IndexOutOfBoundsException(outOfBoundsMsg(index)); } private String outOfBoundsMsg(int index) { return "Index: "+index+", Size: "+size; } private void checkForComodification() { if (this.modCount != l.modCount) throw new ConcurrentModificationException(); } } class RandomAccessSubList<E> extends SubList<E> implements RandomAccess { RandomAccessSubList(AbstractList<E> list, int fromIndex, int toIndex) { super(list, fromIndex, toIndex); } public List<E> subList(int fromIndex, int toIndex) { return new RandomAccessSubList<>(this, fromIndex, toIndex); } }
6. AbstractSet
AbstractSet的定义如下:
public abstract class AbstractSet<E> extends AbstractCollection<E> implements Set<E> {}
AbstractSet是一个继承与AbstractCollection,并且实现了Set接口的抽象类。由于Set接口和Collection接口中的API完全一样,所以Set也就没有自己单独的API。和AbstractCollection一样,它实现了List中除iterator()和size()外的方法。所以源码和AbstractCollection的一样。
AbstractSet的主要作用:它实现了Set接口总的大部分函数,从而方便其他类实现Set接口。
7. Iterator
Iterator的定义如下:
public interface Iterator<E> {}
Iterator是一个接口,它是集合的迭代器。集合可以通过Iterator去遍历其中的元素。Iterator提供的API接口包括:是否存在下一个元素,获取下一个元素和删除当前元素。
注意:Iterator遍历Collection时,是fail-fast机制的。即,当某一个线程A通过iterator去遍历某集合的过程中,若该集合的内容被其他线程所改变了,那么线程A访问集合时,就会抛出CurrentModificationException异常,产生fail-fast事件。下面是Iterator的几个API。
// Iterator的API abstract boolean hasNext() abstract E next() abstract void remove()
8. ListIterator
ListIterator的定义如下:
public interface ListIterator<E> extends Iterator<E> {}
ListIterator是一个继承Iterator的接口,它是队列迭代器。专门用于遍历List,能提供向前和向后遍历。相比于Iterator,它新增了添加、是否存在上一个元素、获取上一个元素等API接口:
// 继承于Iterator的接口 abstract boolean hasNext() abstract E next() abstract void remove() // 新增API接口 abstract void add(E object) abstract boolean hasPrevious() abstract int nextIndex() abstract E previous() abstract int previousIndex() abstract void set(E object)