Question

Ok, I’m semi new to java and I am making a pong game. I want to do it completely by myself, but I’ve come to a problem. I have got 2 classes so far. My main one, and one which contains information about the ball.
My main class is as follows:

    import java.awt.Canvas;
import java.awt.Color;
import java.awt.DisplayMode;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.image.BufferStrategy;

import javax.swing.JFrame;


public class Main extends Canvas implements Runnable{

    private static final long serialVersionUID = 1L;

    public static int Width=800;
    public static int Height=600;
    public boolean Running=false;
    public Thread thread;
    public Ball ball;
    public int BallX = ball.BallLocationX;
    public int BallY = ball.BallLocationY;



    public static void main(String[] args){
        Main game = new Main();
        JFrame frame = new JFrame();
        frame.setSize(Width, Height);
        frame.setTitle("Pong By Poo");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setResizable(false);
        frame.setVisible(true);
        frame.setLocationRelativeTo(null);
        frame.add(game);
        game.start();
        }


    public void start(){
        if(Running==true){
            return;
        }
        else {
            Running=true;
            thread = new Thread(this);
            thread.start();
        }
    }



    public void run(){
        while(Running==true){
            Draw();
        }

    }

    public void Draw(){
        BufferStrategy bs = this.getBufferStrategy();
        if(bs==null){
            createBufferStrategy(2);
        }else{
            Graphics g = bs.getDrawGraphics();
            g.setColor(Color.BLACK);
            g.fillOval(BallX, BallY, 10, 10);   
        }


    }

}

And my ball class is this:

    **import java.awt.Color;
import java.awt.Graphics;

import javax.swing.JPanel;

public class Ball extends JPanel{
    public int BallLocationX;
    public int BallLocationY;
    public boolean BallMovementY; //true makes the ball go up, false makes it go down
    public boolean BallMovementX; //True makes the ball go right, false makes it go left.

    public void paintComponent(Graphics g){
        super.paintComponent(g);
        g.setColor(Color.BLACK);
        g.fillOval(BallLocationX, BallLocationY, 10, 10);
    }

    //moves the ball left to right
    public int YVelocity(){

        if(BallMovementY==true){
            BallLocationY++;
        }
        else{
            BallLocationY--;
        }

        return BallLocationY;

    }

    //Moves the ball up and down
    public int XVelocity(){

        if(BallMovementX==true){
            BallLocationX++;
        }
        else{
            BallLocationX--;
        }

        return BallLocationX;

    }
}
**

Im trying to draw the ball on the screen inside my main class, useing the location of the ball which I get from the ball class.
I know that (As of yet) The ball wont move, Ill figure that out later. My probmlem is that it wont draw the ball on screen, getting me this error:

Exception in thread "main" java.lang.NullPointerException
    at Main.<init>(Main.java:20)
    at Main.main(Main.java:26)

Thanks!

Answer 1

public Ball ball;  // ball is not initialized
public int BallX = ball.BallLocationX;  // NPE here
public int BallY = ball.BallLocationY;

Here’s the problem. In your instance variable declaration, your ball is still pointing to null and you have used it to access BallLocationX. It will throw a NPE.

You should initialize your ball reference to point to an instance of Ball first: –

public Ball ball = new Ball();  // Or whatever way you use to instantiate it
public int BallX = ball.BallLocationX;
public int BallY = ball.BallLocationY;

---

An Advice : –

• I just noticed that, you have used public modifier for all your
  fields. You should not do that. As far as possible, try to have
  private modifier for your fields, and provide public accessors to
  access them. (getters and setters).
• Also, follow Java Naming Conventions. Variable names should start with lowercase alphabets/underscore/dollar symbol (Change BallX to ballX and BallLocationX to ballLocationX)