ConcurrentModificationException 逻辑分解

Question

根据文档 ConcurrentModificationException 指出： ConcurrentModificationException 发生在不允许同时修改对象时。

我正在尝试对某些 java 概念进行除锈并制作霍夫曼压缩应用程序。
我有一个辅助函数，我认为这是造成这种情况的原因，但我不确定为什么。

当我传入 root 并将其设置为由 huffmanHelper

返回的新根时

(我的代码可能还没有完全按照我的要求去做)
我的问题： * 在幕后为什么我正在尝试 java 中的问题。*

谢谢

package huffman;

import java.util.Arrays;
import java.util.HashMap;
import java.util.Map;
import java.util.PriorityQueue;
import java.util.Scanner;

public class huffman {

public static Map<Character, Integer> frequency = new HashMap<Character, Integer>();
public static PriorityQueue<node> nodesPQ = new PriorityQueue<node>();

public static void main(String[] args) {
    
    /* get input */
    Scanner scanner = new Scanner(System.in);
    String input = scanner.nextLine();
    frequency = inputFrequency(frequency,input);
    System.out.println(frequency);
    scanner.close();
    nodesPQ = createNodes(nodesPQ, frequency);
    node a = nodesPQ.poll();
    node b = nodesPQ.poll();
    node OGroot = new node(a,b,a.getFrequency() + b.getFrequency(),null);
    node finalRoot = createBranch(nodesPQ,OGroot);
    finalRoot.inorderPrint();
}


/* counts occurrences of letters in output returns Map<Char, # of occurrences> */
public static Map<Character,Integer> inputFrequency(Map<Character,Integer> map,String input){
    /* split input into char Array */
    char[] charsArray = input.toCharArray();
    
    /* fill hashmap ['Char', # of occurrences] */
    for(char i : charsArray) {
        int count = 0;                                                          
        for(char j : charsArray){
            if(i == j) {
             count++;            
            }
            map.put(i,count);                   
        }
    }
    return map;
}


/* take Map of <Character, # of occurrences> and create nodes inside PQ*/
public static PriorityQueue<node> createNodes(PriorityQueue<node> nodePQ,Map<Character,Integer> map){
            
        nodePQ = new PriorityQueue<>();  
        // create nodes inside PQ
        for (Map.Entry<Character,Integer> i : frequency.entrySet())  {
            Character character = i.getKey();
            Integer occurrences = i.getValue();
            node n = new node(null,null,occurrences,character);
            
                nodePQ.add(n);
            }        
      
        return nodePQ;
        } 


 



public static node createBranch(PriorityQueue<node> nodePQ, node root){
    node newRoot = null;
    for(node i : nodePQ) {
        node nextFrequent = nodePQ.poll();
        root = huffmanHelper(nodesPQ,root,nextFrequent);

    }
        return newRoot;
        
}
    





public static node huffmanHelper(PriorityQueue<node> nodePQ, node root, node nextFrequent){
    node newRoot = new node(nextFrequent,root,root.getFrequency() + nextFrequent.getFrequency(),null);
    
    //get next letter and frequency 
    return newRoot; 
        }   
}

我的堆栈跟踪：

hello my name is sam
{ =4, a=2, s=2, e=2, h=1, y=1, i=1, l=2, m=3, n=1, o=1}
Exception in thread "main" java.util.ConcurrentModificationException
    at java.util.PriorityQueue$Itr.next(PriorityQueue.java:536)
    at huffman.huffman.createBranch(huffman.java:83) - is the start of 
    forLoop in createBranch
    at huffman.huffman.main(huffman.java:26) - is node finalRoot = 
    createBranch(nodesPQ,OGroot); in main

Answer 1

此行从优先级队列中删除一个元素：

    node nextFrequent = nodePQ.poll();

它发生在遍历相同优先级队列的循环中，因此算作禁止的并发修改。这会导致您的异常。

is there a better way you can think of iterating over a PriorityQueue to remove items? just a regular for loop w PQ.size() ?

如果想按优先级排完队列，一直取出元素直到poll() returns null（代码未测试）：

    node nextFrequent = nodePQ.poll();
    while (nextFrequent != null) {
        // Do something with nextFrequent
        nextFrequent = nodePQ.poll();
    }

poll 方法 检索并移除此队列的头部，或者 returns null 如果此队列为空。

文档 link：Queue.poll()

ConcurrentModificationException 逻辑分解

ConcurrentModificationException logic breakdown

java

exception

huffman-code

concurrentmodification