ArrayList的包里面的关于 Iterator 的实现源码：

/**
     * An optimized version of AbstractList.Itr
     */
    private class Itr implements Iterator<E> {
        int cursor;       // 下一个元素的下标
        int lastRet = -1; // 最后一个返回的下标,如果没有,返回 -1
        int expectedModCount = modCount;

        Itr() {}//默认的无参构造方法

        public boolean hasNext() {
            return cursor != size;//cursor 未到达线性表的末尾
        }

        @SuppressWarnings("unchecked")
        public E next() {//获取 list 下一个元素
            checkForComodification();//检查是否有并发的线程对list进行修改
            int i = cursor;
            if (i >= size)
                throw new NoSuchElementException();//越界查询
            Object[] elementData = ArrayList.this.elementData;
            if (i >= elementData.length)
                throw new ConcurrentModificationException();//遇到来自其他线程的修改
            cursor = i + 1;
            return (E) elementData[lastRet = i];
        }

        public void remove() {//移除元素
            if (lastRet < 0)
                throw new IllegalStateException();//非法状态异常
            checkForComodification();

            try {
                ArrayList.this.remove(lastRet);
                cursor = lastRet;
                lastRet = -1;
                expectedModCount = modCount;//修改线性表的大小
            } catch (IndexOutOfBoundsException ex) {
                throw new ConcurrentModificationException();
            }
        }

        @Override
        @SuppressWarnings("unchecked")
        public void forEachRemaining(Consumer<? super E> consumer) {
            Objects.requireNonNull(consumer);
            final int size = ArrayList.this.size;
            int i = cursor;
            if (i >= size) {
                return;
            }
            final Object[] elementData = ArrayList.this.elementData;
            if (i >= elementData.length) {//检查到并发线程的修改
                throw new ConcurrentModificationException();
            }
            while (i != size && modCount == expectedModCount) {
                consumer.accept((E) elementData[i++]);//遍历线性表
            }//在迭代的末尾更新一次，减少堆内存的写入压力
            cursor = i;
            lastRet = i - 1;
            checkForComodification();
        }

        final void checkForComodification() {//检查是否有并发线程的修改，如果有就抛出异常
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
        }
    }