迭代器的原理

1.迭代器几个方法的实现原理：主要使用游标来记录；

1.1数组列表的方式：可以使用一个游标来指向数组的头部，hashnext就是查看判断游标的大小是否大于数组的大小，如果小于hashnext'就会移动游标；在linklist中的实现就更加简单了；

1.2hashSet中的迭代器中使用双重for循环分别遍历数组，以及链表；

下面是迭代器在ArrayList中的实现：

private class Itr implements Iterator<E> {
        int cursor;       // index of next element to return
        int lastRet = -1; // index of last element returned; -1 if no such
        int expectedModCount = modCount;

        // prevent creating a synthetic constructor
        Itr() {}

        public boolean hasNext() {
            return cursor != size;//通过size来确定集合是否遍历完成
        }

        @SuppressWarnings("unchecked")
        public E next() {
            checkForComodification();
            int i = cursor;
            if (i >= size)//抛出没有这个元素异常
                throw new NoSuchElementException();
            Object[] elementData = ArrayList.this.elementData;//获得存储元素的数组
            if (i >= elementData.length)//抛出并发修改异常
                throw new ConcurrentModificationException();
            cursor = i + 1;//游标增加1
            return (E) elementData[lastRet = i];//当前游标返回出去就是next
        }

        public void remove() {
            if (lastRet < 0)
                throw new IllegalStateException();
            checkForComodification();

            try {
                ArrayList.this.remove(lastRet);
                cursor = lastRet;
                lastRet = -1;
                expectedModCount = modCount;
            } catch (IndexOutOfBoundsException ex) {
                throw new ConcurrentModificationException();
            }
        }

        @Override
        public void forEachRemaining(Consumer<? super E> action) {
            Objects.requireNonNull(action);
            final int size = ArrayList.this.size;
            int i = cursor;
            if (i < size) {
                final Object[] es = elementData;
                if (i >= es.length)
                    throw new ConcurrentModificationException();
                for (; i < size && modCount == expectedModCount; i++)
                    action.accept(elementAt(es, i));
                // update once at end to reduce heap write traffic
                cursor = i;
                lastRet = i - 1;
                checkForComodification();
            }
        }

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