java集合框架02——Collection架构与源码分析

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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> {}

    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()

    List的定义如下：

public interface List<E> extends Collection<E> {}

    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

    Set的定义如下：

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

    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> {}

    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：

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> {}

7. Iterator

    Iterator的定义如下：

public interface Iterator<E> {}

    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> {}

// 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)

java集合框架02——Collection架构与源码分析

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原文地址：http://blog.csdn.net/eson_15/article/details/51139978