ArrayList使用foreach循环删除元素为啥报错Exception in thread “main“ java.util.ConcurrentModificationException

List<String> list = new ArrayList<>();
        list.add("1");
        list.add("2");
        list.add("3");
        list.add("4");
        list.add("5");
 for (String s:list) {
            if(s.equals("5")){
                list.remove(s);
            }
      }
    System.out.println(list);

foreach循环实际上是迭代器循环

public E next() {
            checkForComodification();
            int i = cursor;
            if (i >= size)
                throw new NoSuchElementException();
            Object[] elementData = ArrayList.this.elementData;
            if (i >= elementData.length)
                throw new ConcurrentModificationException();
            cursor = i + 1;
            return (E) elementData[lastRet = i];
        }
   final void checkForComodification() {
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
        }

public boolean remove(Object o) {
        if (o == null) {
            for (int index = 0; index < size; index++)
                if (elementData[index] == null) {
                    fastRemove(index);
                    return true;
                }
        } else {
            for (int index = 0; index < size; index++)
                if (o.equals(elementData[index])) {
                    fastRemove(index);
                    return true;
                }
        }
        return false;
    }
      private void fastRemove(int index) {
        modCount++;
        int numMoved = size - index - 1;
        if (numMoved > 0)
            System.arraycopy(elementData, index+1, elementData, index,
                             numMoved);
        elementData[--size] = null; // clear to let GC do its work
    }

从中可以看出每次remove操作会修改modCount的值
该值表示对List的修改次数，查看ArrayList的add()和remove()方法就可以发现，每次调用add()方法或者remove()方法就会对modCount进行加1操作。
expectedModCount：表示对ArrayList修改次数的期望值，它的初始值为modCount