Java中ArrayList和SubList的坑面试题

        List<String> list1 = new ArrayList<>(Arrays.asList("username", "passwd"));
        List<String> list2 = list1.subList(0, 2);
        list2.add("email");
        System.out.println(list1);
        System.out.println(list2);

List<E> subList(int fromIndex, int toIndex);

    public List<E> subList(int fromIndex, int toIndex) {
    subListRangeCheck(fromIndex, toIndex, size);
    return new SubList(this, 0, fromIndex, toIndex);
}

    static void subListRangeCheck(int fromIndex, int toIndex, int size) {
    	//开始索引小于0
        if (fromIndex < 0)
            throw new IndexOutOfBoundsException("fromIndex = " + fromIndex);
            //结束索引大于容量
        if (toIndex > size)
            throw new IndexOutOfBoundsException("toIndex = " + toIndex);
            //开始索引大于结束索引
        if (fromIndex > toIndex)
            throw new IllegalArgumentException("fromIndex(" + fromIndex +
                                               ") > toIndex(" + toIndex + ")");

        SubList(AbstractList<E> parent,
                int offset, int fromIndex, int toIndex) {
             //问题就出现在这里
            this.parent = parent;
            this.parentOffset = fromIndex;
            this.offset = offset + fromIndex;
            this.size = toIndex - fromIndex;
            this.modCount = ArrayList.this.modCount;
        }

        public void add(int index, E e) {
            rangeCheckForAdd(index);
            checkForComodification();
            //将元素直接添加在list1和list2共同的list引用对象上
            parent.add(parentOffset + index, e);
            this.modCount = parent.modCount;
            this.size++;
        }