11.6迭代器

迭代器----是一个对象，他的工作是遍历并选择序列中的对象，而客户端的程序员不必知道和关心该序列底层的结构。迭代器通常被称为‘轻量级对象’，创建他的代价小。

AbstractList下的iterator()方法ArrayList就是继承这个方法：

public Iterator<E> iterator() {
    return new Itr();
    }

 

AbstractList下的内部类 Itr：

private class Itr implements Iterator<E> {
    /**
     * Index of element to be returned by subsequent call to next.
     */
    int cursor = 0;

    /**
     * Index of element returned by most recent call to next or
     * previous.  Reset to -1 if this element is deleted by a call
     * to remove.
     */
    int lastRet = -1;

    /**
     * The modCount value that the iterator believes that the backing
     * List should have.  If this expectation is violated, the iterator
     * has detected concurrent modification.
     */
    int expectedModCount = modCount;

    public boolean hasNext() {
            return cursor != size();
    }

    public E next() {
            checkForComodification();
        try {
        E next = get(cursor);
        lastRet = cursor++;
        return next;
        } catch(IndexOutOfBoundsException e) {
        checkForComodification();
        throw new NoSuchElementException();
        }
    }

    public void remove() {
        if (lastRet == -1)
        throw new IllegalStateException();
            checkForComodification();

        try {
        AbstractList.this.remove(lastRet);
        if (lastRet < cursor)
            cursor--;
        lastRet = -1;
        expectedModCount = modCount;
        } catch(IndexOutOfBoundsException e) {
        throw new ConcurrentModificationException();
        }
    }

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