[Solved] Can't fix this error

Newbie & Debugging Questions
#1

Hello everyone,
this my real first OOP game, i used to do all my programs in one class, i followed some tutorials and then tried to remake a pong game that i already made but everything was in one class, but i don’t want to do that anymore, things becomes really complicated when i want to fix or re-do something with it, i always get lost in the code,
so,
here is all the codes for the new pong and the error is :

[quote]Exception in thread “AWT-EventQueue-0” java.lang.NullPointerException
at Screen.paintComponent(Screen.java:28)
[/quote]
Frame.Java


import java.awt.Dimension;

import javax.swing.JFrame;


public class Frame extends JFrame  {

	public static Dimension size = new Dimension(800,600);
	public static String title ="Pong2.0";
	public Frame(){
		
		this.setSize(size);
		this.setTitle(title);
		this.setLocationRelativeTo(null);
		this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		
		init();
		
	}
	
	public void init(){
		
		Screen screen = new Screen();
		
		this.add(screen);
		
		
		
		this.setVisible(true);
		
	}
	
	
	
	
	public static void main(String args[]){
		Frame frame = new Frame();
	}
	
}

Screen.java

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

import javax.swing.JPanel;


public class Screen extends JPanel implements Runnable {
	public static Thread thread = new Thread();
	
	public static Ball ball;

	
	public Screen(){
		thread.start();
	}

	public void define(){
		
		ball = new Ball(0, 0, 50, 50);
	}
	
	public void paintComponent(Graphics g){
		
		
		g.setColor(new Color(0,0,0));
		g.fillRect(0, 0, Frame.size.width, Frame.size.height);
		
		g.setColor(new Color(0,50,0));
		ball.draw(g);
		
	}
	
	
	@Override
	public void run() {
		while(true){
			repaint();
			
			
			try {
				Thread.sleep(1);
			} catch (InterruptedException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			}
		}
	}
	
}

Ball.java



import java.awt.*;


public class Ball extends Rectangle {
	
	
	public Ball(int x,int y,int width,int height){
		this.setBounds(x, y, width, height);
		
		
	}

	public void draw (Graphics g){
		
		g.drawRect( x, y, width,height);
		
		
	}
}

thank you very much for your help

#2

That’s some crazy OO style you have there.

You are constructing Screen, which starts the thread immediately. This will then reference frame, which has likely not yet actually been assigned.

Cas :slight_smile:

#3

The Ball class member in your Screen class is never created which causes the NPE I think. You have a define() method to create it but it never gets called as far as I can tell.

EDIT: Although I would have then expected the error to report line 29 in that case, if cas in the previous post is correct it would be line 27, odd :clue:

#4

+1 ::slight_smile:
as i said it’s my first time :-
advice, articles, suggestions are very welcome ;D

#5

i don’t understand ???

#6

I’ve highlighted the offending lines, assuming I correctly identified the problem (I didn’t try to compile/run the code).

public class Screen extends JPanel implements Runnable {
public static Thread thread = new Thread();

public static Ball ball; <-- this is the class member that never gets created

public Screen(){
thread.start();
}


This method doesn’t seem to get called anywhere, so ‘ball’ is null.
public void define(){
ball = new Ball(0, 0, 50, 50);
}

public void paintComponent(Graphics g){

  g.setColor(new Color(0,0,0));
  g.fillRect(0, 0, Frame.size.width, Frame.size.height);
  
  g.setColor(new Color(0,50,0));

ball.draw(g); <-- which causes the null pointer exception (NPE) here (I think)

}

@Override
public void run() {
while(true){
repaint();

     try {
        Thread.sleep(1);
     } catch (InterruptedException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
     }
  }

}
}

What are you using to develop the code? Do you have a debugger so you can step through, that’s the best way of tracking down bugs?

Hope this helps.

  • stride

EDIT: Why can’t I make stuff bold within a code block ffs :wink:

#7

[quote]
This method doesn’t seem to get called anywhere, so ‘ball’ is null.
public void define(){
ball = new Ball(0, 0, 50, 50);
}

[/quote]
Biiiiiiig thanx for you :slight_smile: !!!
and even bigger oops for me :stuck_out_tongue:

thank you very much