我无法弄清楚为什么我有这个错误:ConcurrentModificationException

Question

我正在编写一个解决Mastermind游戏的程序.该程序的要点是列出所有可能的解决方案,并在每次猜测不正确之后,从列表中删除任何不至少提供该解决方案的内容.此方法用于比较两个字符串(guess和strFromArray)以查看它们是否获得相同的值.但是,我收到了一个错误,我无法弄清楚原因.任何帮助,将不胜感激.

    Exception in thread "main" java.util.ConcurrentModificationException
at java.util.ArrayList$Itr.checkForComodification(Unknown Source)
at java.util.ArrayList$Itr.next(Unknown Source)
at Game.shortenTheList(Game.java:88)
at Game.guess(Game.java:76)
at Game.play(Game.java:40)
at Game.main(Game.java:23)


/*
 * Compares the two strings. If they would get the same output, return false. If they would get a different output, return true.
 */
public boolean compare(String guess, String strFromArray, int black, int white)
{
    int white2 = 0;
    int black2 = 0;

    char[] arr1 = guess.toCharArray();
    char[] arr2 = strFromArray.toCharArray();

    for(int i=0; i<guess.length(); i++)
    {
        if(arr1[i] == arr2[i])
        {
            black2 ++;
            arr1[i] = '$';
            arr2[i] = '%';
        }
    }

    for(int i=0; i<guess.length(); i++)
    {
        for(int j=0; j<strFromArray.length(); j++)
        {
            if(arr1[i] == arr2[j])
            {
                white2++;
                arr1[i] = '!';
                arr2[j] = '@';
            }
        }
    }

    if(black == black2 && white == white2)
        return false;

    else
        return true;
}

/*
 * Shortens the list of possible solutions by eliminating everything that wouldn't get at least the given output.
 */
public void shortenTheList(String guess, int black1, int white1)
{
    for (String str : possibleSolutions)
    {
        if(compare(guess, str, black1, white1))
        {
            possibleSolutions.remove(str);
        }
    }
}

Answer 1

一旦你打开一个Iterator(你做隐式使用时for(String str: possibleSolutions),向底层集合的任何修改(possibleSolutions)除了调用remove上Iterator会造成ConcurrentModificationException;这非常清楚地记录在集合类.

如果您需要从集合中删除项目,请使用以下内容Iterator:

Iterator<String> it = possibleSolutions.iterator();
while(it.hasNext()) {
    if(compare(guess, it.next(), black1, white1))
        it.remove();
}

正如@allprog指出的那样,当你有一个明确的"过滤"问题时,功能方法会更好.直到Java 8是一个选项,使用Guava Iterables#filter或者Iterables#removeIf可能是一个不错的选择; 你只需要包装你的compare方法并传递它.