Java foreach ConcurrentModificationException解读

这里foreach指的是 for (int num: nums)这种用法

下面的代码

//BFS
List<Node> current = new ArrayList<>();
List<Node> next = new ArrayList<>();

while (!current.isEmpty()) {
   for (Node node : current) {
      next.add(node.left);  //ommit if not null
      next.add(node.right); //ommit if not null
   }
   current.clear();
   current = next;  //want to swap current and next, but mistake
}   

这段代码运行后会出现ConcurrentModificationException
问题出在将current指向next的object，
在foreach遍历current中，next做了修改，因为current也指向next，所以意味着current也在做修改
而foreach是基于iterator实现的，详细参照
一不小心就让Java开发者踩坑的fail-fast是个什么鬼？

抛出CME的地方是

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

可以看到当modCount != expectedModCount时抛出CME

而看ArrayList的底层代码发现，在add的时候modCount会+1
因此导致了上述问题

    /**
     * Appends the specified element to the end of this list.
     *
     * @param e element to be appended to this list
     * @return <tt>true</tt> (as specified by {@link Collection#add})
     */
    public boolean add(E e) {
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;
        return true;
    }