Java 递归迭代时的ConcurrentModificationException

我正在写一个函数，它将使用给定文件夹中的子文件夹ID获取子文件夹列表。不知何故，我最终得到了一个java.util.ConcurrentModificationException

private List<Long> getChildrenFolders(Long folderId) {
        List<Long> directChildren=new CopyOnWriteArrayList <>();
        Iterator<Folder> folderList= (Iterator<Folder>) folderMap.values().iterator();

        while (folderList.hasNext()){
            Folder currentFolder=folderList.next();
            Long parentId=currentFolder.getParentId();
            Long currentFolderId=currentFolder.getId();
            if(parentId.equals(folderId)){
                directChildren.add(currentFolderId);
                Folder folder=  actionManager.getFolderById(currentFolderId);
                folder.getParentId();
            }
        }

        CopyOnWriteArrayList<Long> copy= new CopyOnWriteArrayList<Long>(directChildren);

        Iterator<Long> directChildrenIterator = copy.iterator();

        while (directChildrenIterator.hasNext()){
            Long value=directChildrenIterator.next();
           directChildren.addall(getChildrenFolders(value));
        }
        return directChildren;
    }

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关于我做错了什么有线索吗？非常感谢您的帮助。

您正在从长列表创建迭代器，然后在迭代器中更新第一个列表。这就是ConcurrentModificationException的原因

private List<Long> getChildrenFolders(Long folderId) {
        List<Long> directChildren=new CopyOnWriteArrayList <>();
        Iterator<Folder> folderList= (Iterator<Folder>) folderMap.values().iterator();

        while (folderList.hasNext()){
            Folder currentFolder=folderList.next();
            Long parentId=currentFolder.getParentId();
            Long currentFolderId=currentFolder.getId();
            if(parentId.equals(folderId)){
                directChildren.add(currentFolderId);
                Folder folder=  actionManager.getFolderById(currentFolderId);
                folder.getParentId();
            }
        }

        CopyOnWriteArrayList<Long> copy= new CopyOnWriteArrayList<Long>(directChildren);

        Iterator<Long> directChildrenIterator = copy.iterator();

        while (directChildrenIterator.hasNext()){
            Long value=directChildrenIterator.next();
           directChildren.addall(getChildrenFolders(value));
        }
        return directChildren;
    }

解决这一问题的一种方法是创建一个单独的多头列表，列出将要添加的多头，尽管成本很低

List toAdd=newarraylist（）；
while（directChildrenIterator.hasNext（））{
Long value=directChildrenIterator.next（）；
toAdd.addall（getChildrenFolders（value））；
}
directChildren.addAll（toAdd）；
返回儿童；