problems with displaying..

[Savanna Ray]

Hello,

I'm trying to display a 3 x 3 grid made up of an array of 9 individual cell objects. I created two classes, one for each cell, and one for the whole board. So far, I can only display the board 'border'. When I try to invoke the method to draw each cell from the Board class, i get a null pointer exception error within the black dos window for some reason?? Would anyone care to take a look and give a little feedback? I would really appreciate it.

Here's the code:

import java.awt.*;
import java.applet.Applet;
import java.awt.event.*;
 
public class Lab111 extends Applet implements MouseListener {
 
    private Board tictac = new Board(3,3);
    private int boardX = 60;
    private int boardY = 30;
    private int boardSize = 250;
 
    public void init() {
 
        this.addMouseListener(this);
    }
 
    public void paint(Graphics g) {
 
        g.drawRect(boardX, boardY, boardSize, boardSize);
        tictac.drawBoard(g);
    }
 
    public void mouseClicked(MouseEvent e) {
 
         int mouseX = e.getX();
         int mouseY = e.getY();
    }
 
    public void  mouseReleased(MouseEvent e) {
    }
 
    public void  mousePressed(MouseEvent e) {
    }
 
    public void  mouseEntered(MouseEvent e) {
    }
 
    public void  mouseExited(MouseEvent e) {
    }
}
 
class Board {
 
    private Cell[][] board = new Cell[3][3];
    private int rowIndex;
    private int colIndex;
    private int boardSize = 250;
    public int xStart = 60;
    public int yStart = 30;
 
    public Board(int rows, int columns) {
 
        Cell[][] board = new Cell[rows][columns];
        for(int row = 0; row < board.length; row++) {
            for(int col = 0; col < board[0].length; col++) {
                board[row][col] = new Cell(xStart + col * boardSize/3, yStart + row * boardSize/3, boardSize/3);
            }
        }
    }
 
    public void drawBoard(Graphics g) {
 
        for(int row = 0; row < board.length; row++) {
            for(int col = 0; col < board[0].length; col++) {
                board[row][col].draw(g);
            }
        }
    }
}
 
class Cell {
 
    private int x;
    private int y;
    private int boardSize;
    private int cellSize;
    private String contents;
 
    public Cell(int x, int y, int size) {
 
        this.x = x;
        this.y = y;
        this.cellSize = size;
 
    }
 
    public void draw(Graphics g) {
 
        g.drawRect(x, y, cellSize, cellSize);
    }
}

Thanks
from BC

[Julien Martin]

Hello Savanna,

Your problem lies in the Board constructor. You have instanciated TWO arrays of cells. One here:

 private Cell[][] board = new Cell[3][3];

and a second here:

Cell[][] board = new Cell[rows][columns];

The first one is an instance variable and the second one a local variable.

The one you are using in the drawBoard method is the instance variable which contains null values hence the null pointer exception you get.

Change your Board class to that:

class Board {
    private Cell[][] board;
 
    private int rowIndex;
 
    private int colIndex;
 
    private int boardSize = 250;
 
    public int xStart = 60;
 
    public int yStart = 30;
 
    public Board(int rows, int columns) {
        board = new Cell[rows][columns];
        for (int row = 0; row < board.length; row++) {
            for (int col = 0; col < board[0].length; col++) {
                board[row][col] = new Cell(xStart + col * boardSize / 3, yStart + row * boardSize / 3, boardSize / 3);
            }
        }
    }
    public void drawBoard(Graphics g) {
        for (int row = 0; row < board.length; row++) {
            for (int col = 0; col < board[0].length; col++) {
                board[row][col].draw(g);
            }
        }
    }
}
 

Tell me if that helped,

Julien.