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Offline Moley

Innocent Bystander





« Posted 2005-07-21 14:01:47 »

Hi, I have just started games programming (well programming in general - read a few books at home etc) , i have read through most of the tutorials here and have ordered a book from amazon to get me started. Well i have a problem...

This is my code so far (yeah its probably s**t)...

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package roadrunner;

import java.awt.*;
import java.awt.event.*;
import java.awt.Graphics;
import java.awt.Dimension;
import java.awt.image.BufferedImage;
import javax.swing.*;
import javax.swing.event.*;

public class MainFrame extends Canvas{
       
    private int carx;
    private int cary;
    private int cardx;
    private int cardy;
   
    private int carwidth;
    private int carheight;
   
    private Image offscreenImage;
    private Graphics offscr;
    private int width = 800, height = 600;
   
    /** Creates a new instance of MainFrame */
    public MainFrame() {
       
        JFrame frame = new JFrame("Road Runner");
       
        JPanel panel1 = (JPanel)frame.getContentPane();
        setBounds(0,0,width,height);
        panel1.setPreferredSize(new Dimension(width,height));
        panel1.setLayout(null);
        panel1.add(this);
       
        carwidth = 30;
        carheight = 30;
        carx = 100;
        cary = 100;
        cardx = 0;
        cardy = 0;
       
        offscreenImage = createImage(width, height);
        offscr = offscreenImage.getGraphics();
       
        frame.setBounds(0,0,width,height);
        frame.pack();
        frame.setResizable(false);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
    }
   
    public void paint(Graphics g){
        offscr.setColor(Color.black);
        offscr.fillRect(0,0, width, height);
       
        offscr.setColor(Color.blue);
        offscr.fillRect(carx, cary, carwidth, carheight);
       
        g.drawImage(offscreenImage, 0, 0, this);
    }
   
    public void updateWorld(){
        carx = (int)(Math.random()*width);
        cary = (int)(Math.random()*height);
    }
   
    public void game(){
        while(isVisible()){
            updateWorld();
            paint(getGraphics());
        }
    }
   
    public static void main(String[] args) {
       
        MainFrame frame1 = new MainFrame();
        frame1.game();
    }
   
}


and whenever i run it i get this error...

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 Exception in thread "main" java.lang.NullPointerException
        at roadrunner.MainFrame.<init>(MainFrame.java:58)
        at roadrunner.MainFrame.main(MainFrame.java:89)
Java Result: 1


...and i cant seem to figure out why?  Oh and this is just to test out problems etc

[EDIT] - I have fixed the problem... it turns out that

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offscreenImage = createImage(width, height);
        offscr = offscreenImage.getGraphics();


should of been place in paint().
Offline oNyx

JGO Coder


Medals: 2


pixels! :x


« Reply #1 - Posted 2005-07-22 10:44:23 »

>should of been place in paint().

Kinda.

offscr = offscreenImage.getGraphics();

Should be in paint() whereas...

offscreenImage = createImage(width, height);

can (should) stay in the ctor.

弾幕 ☆ @mahonnaiseblog
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