我不明白爲什麼我有這個錯誤：ConcurrentModificationException

Question

我正在寫一個解決Mastermind遊戲的程序。該計劃的要點是要列出所有可能的解決方案，並在每次猜測不正確之後，從列表中刪除任何至少不會提供該解決方案的內容。這個方法用來比較兩個字符串（guess和strFromArray），看它們是否得到相同的值。但是，我收到一個錯誤，我無法弄清楚爲什麼。任何幫助，將不勝感激。

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

一旦你打開一個Iterator（使用for(String str: possibleSolutions)，底層集合（possibleSolutions）任何修改時除外通過調用Iteratorremove你做隱含會造成ConcurrentModificationException，這是非常清楚地記錄在採集班

如果您需要從集合中刪除的項目，使用一個明確的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，當你有這樣一個明確的「過濾」問題時，功能性方法會更好。在選擇Java 8之前，使用Guava的Iterables#filter或Iterables#removeIf可能是一個不錯的選擇;你只需將你的compare方法包裝起來並將其傳入。

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); 
     } 
    } 
} 

這是你的問題。

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(); 
     } 
    } 
} 

這是唯一干淨的方式從你目前遍歷集合中刪除的對象。另一種較不優雅的方式是創建一個單獨的String列表來刪除和迭代該列表。

Answer 3

當您使用foreach循環遍歷它時，您無法修改列表possibleSolutions。

更改您的代碼是這樣的：

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說，你不能沒有使用迭代器中刪除從集合的元素。

例如：

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

應該是：

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(); 
} 
} 
}