iterator

核心变量：

• cursor：指向下一个将要被访问的元素的索引号
• lastRet：指向刚被访问的元素的索引号
• expectedModCount：通过与ModCount比较，判断是不是进行了iterator内部remove的增删操作

核心方法：

• hasNext（）：比较cursor与size（）
• next（）：返回下一个元素
• remove：删除集合中的元素，并更新expectedModCount和ModCount
• checkForCoModComodification（）：检测expectedModCount和ModCount是否相等

Iterable接口与Iterator接口

• Iterable接口只有一个iterator（）方法，用于返回实现了Iterator接口的类
• Iterator接口规定了操作集合元素的方法
  • hasNext（）
  • next（）
  • remove（）

源码：来自AbstractList<E>的Itr类，这个类尤iterator（）方法返回

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 {                int i = cursor;                E next = get(i);                lastRet = i;                cursor = i + 1;                return next;            } catch (IndexOutOfBoundsException e) {                checkForComodification();                throw new NoSuchElementException();            }        }        public void remove() {            if (lastRet < 0)                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();        }    }