Case specific null pointer exception problem - java

Question

I have a strange problem regarding null pointer exception. I'm posting the problematic code piece below :

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
    }

...

}

Thanks in advance for your time.

Can you post the stack trace? Is the problem showing up with that specific data when calling `addResponse()` (`2,1,0`) — Argote, Feb 28 '11 at 16:57

score 3 · Accepted Answer · answered Feb 28 '11 at 16:55

In this line:

responses = new PlayerResponses[4][totalPlayers];

You create a 2-dimensional array that can hold references to PlayerResponses objects. You don't create even a single PlayerResponses object.

You need to fill in those in a loop (or later on on-demand, if you want):

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

score 0 · Answer 2 · answered Feb 28 '11 at 16:58

0

That's because public List<Integer> response; in each of the PlayerResponses stored in the array are not initialized. It gets the default null, that's why when you call the add method you get the NPE.

answered Feb 28 '11 at 16:58

reef

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score 0 · Answer 3 · answered Feb 28 '11 at 17:02

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
        }


    }

I don't think that spoon feeding (especially when there's no explanation) is the solution here. — Joachim Sauer, Feb 28 '11 at 17:04

Case specific null pointer exception problem - java

3 Answers3