在同一arraylist上使用addall和removeall的并发修改异常 [英] concurrent modification exception on using addall and removeall on same arraylist

问题描述

我必须从 ArrayList 中删除一个子列表，并在开始时添加一个子列表.我正在使用以下代码-

I have to remove a sublist from the ArrayList and add the same in begining. I am using below code -

for(int i =0 ;i <m ; i++){
    List<Integer> t = q.subList(j, k);
    q.removeAll(t);
    q.addAll(0, t);
}

我正在 addAll 上进行并发异常修改.我尝试从q创建一个副本列表，然后在其上调用 addAll ，但仍然在同一行上引发错误.

I am getting concurrent exception modification on addAll. I tried creating a copy list from q and then calling addAll on that, still it throws the error on same line.

for(int i =0 ;i <m ; i++){
    List<Integer> t = q.subList(j, k);
    q.removeAll(t);
    ArrayList<Integer> q1= new ArrayList<Integer>(q);
    q1.addAll(0, t);
}

如何在同一数据列表上执行这两项操作?

How can I perform both actions on same data list?

推荐答案

subList 的Javadoc指出:

The Javadoc of subList states that:

如果后备列表(即此列表)以结构方式(而不是通过返回的列表)进行了修改，则此方法返回的列表的语义将变得不确定.(结构修改是指更改此列表的大小的结构修改，或以其他方式扰乱此列表的方式，以至于正在进行的迭代可能会产生错误的结果.)

The semantics of the list returned by this method become undefined if the backing list (i.e., this list) is structurally modified in any way other than via the returned list. (Structural modifications are those that change the size of this list, or otherwise perturb it in such a fashion that iterations in progress may yield incorrect results.)

这意味着调用 removeAll 可能会使 subList 返回的列表无效.

This means calling removeAll may invalidate the list returned by subList.

此外， addAll 的Javadoc指出:

In addition, the Javadoc of addAll states that:

如果在操作进行过程中修改了指定的集合，则此操作的行为是不确定的.(请注意，如果指定的集合是此列表，并且是非空的，则将发生.)

在您的情况下，您要传递给 addAll 的 Collection 是子列表，该列表由原始的 List 支持您正在呼叫 addAll .

And in your case, the Collection you are passing to addAll is the sub-list, which is backed by the original List on which you are calling addAll.

您可以做的是将子列表的元素存储在单独的 List ( copyOfSubList )中，使用 clear()删除子列表的元素(这也会从原始的 List 中删除它们)，然后将 copyOfSubList 的元素添加回原始的 List :

What you can do is store the elements of the sub-list in a separate List (copyOfSubList), use clear() to remove the elements of the sub-list (which will remove them also from the original List) and then add the elements of copyOfSubList back to the original List:

for(int i =0 ;i < m ; i++) {
    List<Integer> t = q.subList(j, k);
    List<Integer> copyOfSubList = new ArrayList<> (t);
    t.clear ();
    q.addAll(0, copyOfSubList);    
}

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