Question

All,

I am trying to code a Connect4 game. For this, I have created a P4Game class and a P4Board class which represents the i X j dimensions of the Connect4 board.

In P4Game, I have the following:

public class P4Game{
  //INSTANCE VARIABLES
  private int nbLines;
  private int nbColumns;
  private P4Board [][] position;  

//CONSTRUCTOR  
  public P4Game(int nbLines, int nbColumns){
    this.nbColumns = nbColumns;
    this.nbLines = nbLines;
    P4Board [][] position = new P4Board [nbLines][nbColumns]; //Creates the table to receive the instances of the P4Board object.*/
    for (int i=0; i<nbLines; i++){
      for (int j=0; j<nbColumns; j++){
        this.position[i][j] = new P4Board(i,j); //Meant to create each object at (line=i, column=j)
      }
    } 
  }

This causes a NullPointerException in the nested loops where I mention this.position[i][j]. I reference those objects in other methods of this class so I need them to be instance variables. I suppose the exception is due to the fact that I have not listed the table element position[i][j] as an instance variable at the beginning of the class.

my question to people here is (1) is my assumption correct, and if so (2) what would be the syntax to declare instance variables of this form?

Thank you all for your help with what I realize is a very basic question. Hopefully it will also benefit other newbies.

Cheers,

JDelage

Answer 1

See added comment inlined… You code is fine except for one little detail, where you’re creating a new position variable where you actually mean to use the instance variable.

public class P4Game{
    //INSTANCE VARIABLES
    private int nbLines;
    private int nbColumns;
    private P4Board [][] position;  

    //CONSTRUCTOR  
    public P4Jeu(int nbLines, int nbColumns){
        this.nbColumns = nbColumns;
        this.nbLines = nbLines;
        // You're creating a LOCAL variable called position here if you don't comment what's commented:.
        /*P4Board [][] */position = new P4Board [nbLines][nbColumns]; //Creates the table to receive the instances of the P4Board object.*/
        for (int i=0; i<nbLines; i++){
            for (int j=0; j<nbColumns; j++){
                this.position[i][j] = new P4Board(i,j); //Meant to create each object at (line=i, column=j)
            }
        } 
    }
}