为什么我会收到 ConcurrentModificationException? [英] Why do I get a ConcurrentModificationException?

问题描述

为什么我在代码中的指定位置会收到 ConcurrentModificationException?我不知道我做错了什么...... removeMin() 方法被用来定位列表 pq 中的最小值，删除它，并返回它的值

Why do I get a ConcurrentModificationException at the specified location in my code? I cannot figure out what I am doing wrong... The removeMin() method is being used to locate the min in the list pq, remove it, and return its value

import java.util.Iterator;
import java.util.LinkedList;

public class test1 {

    static LinkedList<Integer> list = new LinkedList<Integer>();

    public static void main(String[] args) {
        list.add(10);
        list.add(4);
        list.add(12);
        list.add(3);
        list.add(7);

        System.out.println(removeMin());
    }

    public static Integer removeMin() {
        LinkedList<Integer> pq = new LinkedList<Integer>();
        Iterator<Integer> itPQ = pq.iterator();

        // Put contents of list into pq
        for (int i = 0; i < list.size(); i++) {
            pq.add(list.removeFirst());
        }

        int min = Integer.MAX_VALUE;
        int pos = 0;
        int remPos = 0;

        while (itPQ.hasNext()) {
            Integer element = itPQ.next(); // I get ConcurrentModificationException here
            if (element < min) {
                min = element;
                remPos = pos;
            }
            pos++;
        }

        pq.remove(remPos);
        return remPos;
    }

}

推荐答案

一旦从中获取迭代器的集合被修改，则不应认为迭代器可用.(这个限制对于 java.util.concurrent.* 集合类放宽了.)

An Iterator should not be considered usable once the Collection from which it was obtained is modified. (This restriction is relaxed for java.util.concurrent.* collection classes.)

您首先为pq 获得一个迭代器，然后修改pq.一旦你修改了 pq，迭代器 itPQ 就不再有效，所以当你尝试使用它时，你会得到一个 ConcurrentModificationException.

You are first obtaining an Iterator for pq, then modifying pq. Once you modify pq, the Iterator itPQ is no longer valid, so when you try to use it, you get a ConcurrentModificationException.

一种解决方案是移动 Iterator;itPQ = pq.iterator(); 在 while 循环之前.更好的方法是完全取消 Iterator 的显式使用:

One solution is to move Iterator<Integer> itPQ = pq.iterator(); to right before the while loop. A better approach is to do away with the explicit use of Iterator altogether:

for (Integer element : pq) {

从技术上讲，for-each 循环在内部使用迭代器，因此无论哪种方式，只要您不尝试修改循环内的 pq，此循环才有效.

Technically, the for-each loop uses an Iterator internally, so either way, this loop would only be valid as long as you don’t try to modify pq inside the loop.

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