我不明白为什么我有这个错误：ConcurrentModificationException

Question

我正在写一个解决Mastermind游戏的程序。该计划的要点是要列出所有可能的解决方案，并在每次猜测不正确之后，从列表中删除任何至少不会提供该解决方案的内容。这个方法用来比较两个字符串（guess和strFromArray），看它们是否得到相同的值。但是，我收到一个错误，我无法弄清楚为什么。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（使用for(String str: possibleSolutions)，底层集合（possibleSolutions）任何修改时除外通过调用Iteratorremove你做隐含会造成ConcurrentModificationException，这是非常清楚地记录在采集班

如果您需要从集合中删除的项目，使用一个明确的Iterator：

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

正如@allprog所指出的，当你有这样一个明确的“过滤”问题时，功能性方法会更好。在选择Java 8之前，使用Guava的Iterables#filter或Iterables#removeIf可能是一个不错的选择;你只需将你的compare方法包装起来并将其传入。

Answer 2

/* 
* 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); 
     } 
    } 
} 

这是你的问题。

相反，使用：

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

这是唯一干净的方式从你目前遍历集合中删除的对象。另一种较不优雅的方式是创建一个单独的String列表来删除和迭代该列表。

Answer 3

当您使用foreach循环遍历它时，您无法修改列表possibleSolutions。

更改您的代码是这样的：

public void shortenTheList(String guess, int black1, int white1) 
{  
    for(Iterator<String> it = possibleSolutions.iterator(); it.hasNext()){ 
     String str = it.next(); 
     if(compare(guess, str, black1, white1)){ 
      it.remove(); 
     } 
    } 
} 

Answer 4

像chrylis说，你不能没有使用迭代器中删除从集合的元素。

例如：

public void shortenTheList(String guess, int black1, int white1) 
{ 
for (String str : possibleSolutions) 
{ 
if(compare(guess, str, black1, white1)) 
{ 
possibleSolutions.remove(str); 
} 
} 
} 

应该是：

public void shortenTheList(String guess, int black1, int white1) 
{ 
Iterator i = possibleSolutions.Iterator(); 
while(i.hasNext()) 
{ 
String str = (String) i.next(); 
if(compare(guess, str, black1, white1)) 
{ 
i.remove(); 
} 
} 
}