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.

Answer 1

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();