特定于案例的空指针异常问题 - java

Question

我有一个关于空指针异常的奇怪问题。我发布下面有问题的代码片：

public static void main(String[] args) { 

BetHistory testObject = new BetHistory(6); 

testObject.addResponse(2, 1, 0); // ERROR HERE 

... 

} 

public class PlayerResponses { 

    public List<Integer> response; 

    public PlayerResponses() { 
     super(); 
     response = new LinkedList<Integer>(); 
    } 

... 

} 

public class BetHistory { 

    PlayerResponses [][] responses; 
    int nPlayer; 

    public BetHistory(int totalPlayers) { 
     super(); 
     responses = new PlayerResponses[4][totalPlayers]; 
     nPlayer = totalPlayers; 

     } 
    public void addResponse(int response, int playerNo, int roundNo) 
    { 
     responses[roundNo][playerNo].response.add(response); // DUE TO HERE 
    } 

... 

} 

在此先感谢您的时间。

Answer 1

在这一行：

responses = new PlayerResponses[4][totalPlayers]; 

您创建一个2维数组，可容纳引用PlayerResponses对象。你不要创建一个单一的PlayerResponses对象。

您需要填写那些在一个循环（或更高版本上的需求，如果你想）：

responses[i][j] = new PlayerResponses(); 

Answer 2

这是因为public List<Integer> response;在每个存储阵列中的PlayerResponses的不被初始化。 它得到默认null，这就是为什么当你打电话给add方法时，你得到NPE。

Answer 3

import java.util.*; 

    public class A { 
    public static void main(String[] args) { 

      BetHistory testObject = new BetHistory(6); 

      testObject.addResponse(2, 1, 0); // ERROR HERE 

    } 
    } 

    class PlayerResponses { 

     public LinkedList<Integer> response; 

     public PlayerResponses() { 
      super(); 
      response = new LinkedList<Integer>(); 
     } 


    } 

    class BetHistory { 

     PlayerResponses [][] responses; 
     int nPlayer; 

     public BetHistory(int totalPlayers) { 
      super(); 
      responses = new PlayerResponses[4][totalPlayers]; 
      for(int i=0;i<totalPlayers;i++) { 
          responses[0][i] = new PlayerResponses(); 
          responses[1][i] = new PlayerResponses(); 
          responses[2][i] = new PlayerResponses(); 
          responses[3][i] = new PlayerResponses(); 
        } 
      nPlayer = totalPlayers; 

      } 
     public void addResponse(int response, int playerNo, int roundNo) 
     { 
      responses[roundNo][playerNo].response.add(response); // DUE TO HERE 
     } 


    }