Question

我有一个关于空指针异常的奇怪问题。我在下面发布有问题的代码：

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
    }

...

}

提前感谢您的时间。

Answer 1

在这一行：

responses = new PlayerResponses[4][totalPlayers];

您可以创建一个可以保存对PlayerResponses个对象的引用的二维数组。您不甚至创建一个PlayerResponses对象。

您需要在循环中填写（或者稍后根据需要填写）：

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

Answer 2

这是因为数组中存储的每个public List<Integer> response;中的PlayerResponses都没有初始化。它获得了默认的null，这就是为什么当您调用add方法时获得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
        }


    }

特定于案例的空指针异常问题 - java

3 个答案: