java - 仅在迭代时如何避免 ArrayList 中的 ConcurrentModificationException？

Question

澄清一下 - 我不想从 ArrayList 中删除任何内容。因此，我找到的所有答案中有 90% 实际上并不适用。我在这里或其他地方找不到任何对我有很大帮助的东西!

我正在编写一个 Java 应用程序来玩 Hangman，其中对手(计算机)本质上是作弊，从某种意义上说，它不“选择”一个单词，它有一组单词并决定玩家的猜测是否正确，或不正确，取决于哪一个留下了更难猜测的单词组。

简而言之，我的问题是这样的:

我有一个 ArrayList，masterList，其中有一组单词、一本字典(如果您愿意的话)以及各种方法迭代它以执行各种任务。我的代码是单线程的，当尝试在第二次迭代中访问 ArrayList 中的下一个对象时，其中一个方法会抛出 ConcurrentModificationException 。但是，我找不到在迭代过程中实际更改 ArrayList 的任何内容。

import java.io.*;
import java.util.*;

public class Main {
    private ArrayList<String> masterList;
    private ArrayList<String> contains;
    private ArrayList<String> doesNotContain;
    private HashMap<Integer, ArrayList<String>> wordLengthList;
    private HashMap<Integer, ArrayList<String>> difficultyList;
    private int guesses = 10;
    private Scanner sc;
    private FileReader fr;
    private BufferedReader br;
    private String guessString;
    private char guessChar;
    private static final String DICTIONARY = "smalldictionary.txt";
    private String wordLengthString;
    private int wordLengthInt = 0;


    public Main(){

        masterList = new ArrayList<String>();
        contains = new ArrayList<String>();
        doesNotContain= new ArrayList<String>();
        wordLengthList = new HashMap<Integer, ArrayList<String>>();
        difficultyList = new HashMap<Integer, ArrayList<String>>();

        sc = new Scanner(System.in);

        importTestDictionary(); //does not use masterList

        br = new BufferedReader(fr);

        importWords(); //Adds to masterList. Both readers closed when finished.

        catalogLengths(); //Iterates through masterList - does not change it.


        do{
            setWordLength(); //does not use masterList
        }while(!(validateLengthInput(wordLengthString))); //validation will change the set of masterList if valid.

        //Main loop of game:
        while(guesses > 0){

            do{
                getUserInput();
            }while(!(validateInput(guessString))); 

            splitFamilies();//will change set of masterList when larger group is found. Changes occur AFTER where Exception is thrown
            printDifficultyList();
        }
    }

    private void importWords(){ //Adds to masterList. Both readers closed when finished.


        try{
            while(br.readLine() != null){
                line = br.readLine();
                masterList.add(line); 
            }
            br.close();
            fr.close();
        }catch(IOException e){
            System.err.println("An unexpected IO exception occurred. Check permissions of file!");
        }
    }


    private boolean validateLengthInput(String length){ //validation will change the set of masterList if valid.
        try{
            wordLengthInt = Integer.parseInt(length);
            if(!(wordLengthList.containsKey(wordLengthInt))){
                System.out.println("There are no words in the dictionary with this length.\n");
                return false;
            }
        }catch(NumberFormatException e){
            System.out.println("You must enter a number.\n");
            return false;
        }
        masterList = wordLengthList.get(wordLengthInt);
        return true;

    }


    private void splitFamilies(){ //will change set of masterList when larger group is found. Changes occur AFTER where Exception is thrown
        Iterator<String> it = masterList.iterator();
        int tempCount = 0;
        while(it.hasNext()){ 
            tempCount++;
            System.out.println("tempCount: " + tempCount);
            String i = it.next(); //Still throwing ConcurrentModification Exception
            if(i.contains(guessString)){
                contains.add(i);
            }else{
                doesNotContain.add(i);
            }
        }

        if(contains.size() > doesNotContain.size()){
            masterList = contains;
            correctGuess(); //does not use masterList
            profileWords();

        }
        else if(doesNotContain.size() > contains.size()){
            masterList = doesNotContain;
            incorrectGuess(); //does not use masterList
        }
        else{
            masterList = doesNotContain;
            incorrectGuess(); //does not use masterList
        }

    }



    private void printMasterList(){ //iterates through masterList - does not change it.
            for(String i : masterList){
                System.out.println(i);
            }
        }


    private void catalogLengths(){ //Iterates through masterList - does not change it.
        for(String i : masterList){
            if(i.length() != 0){
                if(!(wordLengthList.containsKey(i.length()))){
                    wordLengthList.put(i.length(), new ArrayList<String>());
                }
                wordLengthList.get(i.length()).add(i);
            }
        }
    }
}

抛出异常的行在代码中标记在上面。任何使用masterList的方法也被标记，任何包含不使用它的方法，没有评论反对。

我确实阅读了一些答案，其中一些建议使用Iterator来避免异常。这是在上面的 splitFamilies() 中实现的。原代码如下:

private void splitFamilies(){ //will change set of masterList when larger group is found. Changes occur AFTER where Exception is thrown
        int tempCount = 0;
        for(String i : masterList){  //This line throws ConcurrentModificationException
            tempCount++;
            System.out.println("tempCount: " + tempCount);
            if(i.contains(guessString)){
                contains.add(i);
            }else{
                doesNotContain.add(i);
            }
        }
....continue as before

抛出异常时，

tempCount 始终为 2。

也许我错过了一些非常简单的东西，但我尝试过追踪它，但无法找出为什么会出现此异常!

我试图删除代码中不相关的所有内容，但如果有人真的想查看完整的内容，我想我可以转储问题中的所有代码!

Answer 1

问题来自于 masterList是对 contains 的引用或doesNotContain第一次 split 后。当您迭代masterList时，您实际上还同时在另一个列表上进行迭代。

那么，您可以将项目添加到列表中:

if(i.contains(guessString)){
    contains.add(i);
}else{
    doesNotContain.add(i);
}

在这里，您不仅可以将项目添加到 contains或doesNotContain ，但也有可能 masterList ，这导致 conccurentException .

<小时/>

要解决您的问题，只需复制您的列表即可，而不是:masterList = contains;
复制一份:masterList = new ArrayList<>(contains);

doesNotContains 也是如此.

<小时/>

想到的另一个解决方案是重置两个列表 contains和doesNotContains对于每个分割。由于您只在这个方法中使用它们，而没有在其他地方使用它们，因此从您的类中删除这两个列表，并将它们定义为 splitFamilies 中的私有(private)变量。