java - 在递归函数中使用一对列表时如何解决 ConcurrentModificationException？

Question

所以我试图通过递归设置每个节点的子节点来平衡二叉搜索树:

public void balanceTheTree(){
    root = balance(keys, names);
}

public Node balance(List<Integer> keyList, List<String> nameList){
    buildArrays(root);
    if(root == null){
        return null;
    }
    else{
        int newRootIndex = keyList.size()/2;
        Node focusNode = new Node(keyList.get(newRootIndex), nameList.get(newRootIndex));
        focusNode.leftChild = balance(keyList.subList(0, newRootIndex), nameList.subList(0, newRootIndex));
        focusNode.rightChild = balance(keyList.subList(newRootIndex+1, keyList.size()),nameList.subList(newRootIndex+1, nameList.size()));
        return focusNode;
    }

}

buildArrays() 使用顺序遍历将树的值(名称)和键(键)存储在各自的列表中。名称是 List<String>键是一个 List<Integer> .

不幸的是，balance 在试图访问列表的任何行都会抛出异常。

这里是错误:

Exception in thread "main" java.util.ConcurrentModificationException
at java.util.ArrayList$SubList.checkForComodification(Unknown Source)
at java.util.ArrayList$SubList.size(Unknown Source)
at BinaryTree.balance(BinaryTree.java:43)
at BinaryTree.balance(BinaryTree.java:45)
at BinaryTree.balanceTheTree(BinaryTree.java:34)
at tester.main(tester.java:30)

第 43 行是 int newRootIndex = keyList.size()/2;

谢谢!

public void buildArrays(Node focusNode){
    if(focusNode != null){
        buildArrays(focusNode.leftChild);

        names.add(focusNode.name);
        keys.add(focusNode.Key);

        buildArrays(focusNode.rightChild);
    }
}

Answer 1

问题是你使用了keyList.subList() ，您要向其中添加新元素，这是 ConcurrentModificationException 的原因.

传递实际子列表的副本，而不是子列表，例如new ArrayList<>(keylist.subList(0, 4)) .