java foreach list.remove()一定会报错吗

来两个例子热身。

        String[] arrays = {"1", "2", "3", "5" ,"4"};
        List<String> list = new ArrayList<>(Arrays.asList(arrays));
        for (String str : list) {
            //remove "1","2","3","4"会报错吗
            if (str.equals("1")) {
                list.remove(str);
            }
        }

答案：会

        String[] arrays = {"1", "2", "3", "5" ,"4"};
        List<String> list = new ArrayList<>(Arrays.asList(arrays));
        for (String str : list) {
            // remove "5"会报错吗？
            if (str.equals("5")) {
                list.remove(str);
            }
        }

答案： 不会

但是不能在foreach里面这样做

fastRemove 修改

 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
    }

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

public boolean hasNext() {
    return cursor != size;
}

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];
}

ArrayList总共有7种remove方法

remove方法的时序图

    public boolean remove(Object o) {
        if (o == null) {
            // 找到为null的索引 i，然后remove(int i);
        } else {
            for (int index = 0; index < size; index++)
                if (o.equals(elementData[index])) {
                    // 此处调用
                    fastRemove(index);
                    return true;
                }
        }
        return false;
    }