offscreen.getGraphics() error

[luis.depedro]

I'm trying to write a simple spaceship game using Java.
It's annoying that images flash when they move on a window.
I have heard about double buffering but I can't get it work.

First I create a program that draws a rectangle which moves when yoy press right and left arrows:

import java.awt.Frame;
import java.awt.event.WindowEvent;
import java.awt.event.WindowListener;
import java.awt.Graphics;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;
class DoubleBuffering {
		public static void main(String[] args)throws InterruptedException{
		InteractiveWindow myWindow=new InteractiveWindow();
		myWindow.setSize(500,400);
		myWindow.setVisible(true);
		myWindow.setResizable(false);
		myWindow.setTitle("Moving Rectangle");
	}
}



class InteractiveWindow extends Frame implements WindowListener, KeyListener{
	Dimension dim;
	int posx=300;
	int posy=300;
	public InteractiveWindow(){
		addKeyListener(this);
		addWindowListener(this);
		
	}
	
	public void paint(Graphics g) {
		g.drawRect(posx,posy,50,50);
	 }

	public void keyPressed(KeyEvent e) {
		int numKey = e.getKeyCode();
		if (numKey==37) {
			posx--;
			repaint();
		}
		if (numKey==39){
			posx++;
			repaint();
		}
		
	}

	public void keyReleased(KeyEvent arg0) {}
	public void keyTyped(KeyEvent arg0) {}
	public void windowActivated(WindowEvent e) {}
	public void windowClosed(WindowEvent e) {System.exit(0);}
	public void windowClosing(WindowEvent e) {System.exit(0);}
	public void windowDeactivated(WindowEvent e) {}
	public void windowDeiconified(WindowEvent e) {}
	public void windowIconified(WindowEvent e) {}
	public void windowOpened(WindowEvent e) {}

}

Then I tryed to introduce an offscreen image to draw on it and then draw it on the Graphics variable of the paint(Graphics g) method, like this: (look at lines 29 and 30)

import java.awt.Frame;
import java.awt.event.WindowEvent;
import java.awt.event.WindowListener;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Image;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;
class DoubleBuffering {
		public static void main(String[] args)throws InterruptedException{
		InteractiveWindow myWindow=new InteractiveWindow();
		myWindow.setSize(500,400);
		myWindow.setVisible(true);
		myWindow.setResizable(false);
		myWindow.setTitle("Moving Rectangle");
	}
}



class InteractiveWindow extends Frame implements WindowListener, KeyListener{
	Dimension dim;
	int posx=300;
	int posy=300;
	Graphics myGraphic;
	Image offscreen;
	public InteractiveWindow(){
		dim=getSize();
		offscreen=createImage(500,400);
		myGraphic=offscreen.getGraphics();
		addKeyListener(this);
		addWindowListener(this);
		
	}
	
	public void paint(Graphics g) {
		myGraphic.drawRect(posx,posy,50,50);
		g.drawImage(offscreen,0,0,this);
	 }

	public void keyPressed(KeyEvent e) {
		int numKey = e.getKeyCode();
		if (numKey==37) {
			posx--;
			repaint();
		}
		if (numKey==39){
			posx++;
			repaint();
		}
		
	}

	public void keyReleased(KeyEvent arg0) {}
	public void keyTyped(KeyEvent arg0) {}
	public void windowActivated(WindowEvent e) {}
	public void windowClosed(WindowEvent e) {System.exit(0);}
	public void windowClosing(WindowEvent e) {System.exit(0);}
	public void windowDeactivated(WindowEvent e) {}
	public void windowDeiconified(WindowEvent e) {}
	public void windowIconified(WindowEvent e) {}
	public void windowOpened(WindowEvent e) {}

}

But I get a NullPointer Exception on line:

myGraphic=offscreen.getGraphics();

What's wrong?

[Norm]

Try using Swing classes instead of AWT classes.
With Swing classes you override the paintComponent method instead of the paint method.

[Tolls]

Another AWT problem.
Is there some bogus tutorial out there that focuses on AWT or something?

[luis.depedro]

When I use JFrame and paintComponent, the rectangle doesn't even appear

[Norm]

Please post the code with the problem.

[DarrylBurke]

When I use JFrame and paintComponent, the rectangle doesn't even appear

I wouldn't expect it to. Custom painting isn't done in a top level window.

Please go through Lesson: Performing Custom Painting (The Java™ Tutorials > Creating a GUI With JFC/Swing)

db

[luis.depedro]

This is why I didnt want to use swing components Its getting more and more complicated.
What should I use to paint on?
A Jpane inside a Jframe? A canvas?

[Norm]

A Jpane inside a Jframe? A canvas?

Neither are Swing classes.
Try a JPanel

[luis.depedro]

Thank you guys. I did it using JPanel.
I had to add a setFocusable() so that it could listen the keboard.
And had to add a super.paintComponent() so that the JPanel might clear everytime.

But at last, it's fine. I don't need double buffering beacause Jpanel implements it.

[Norm]

Glad you got it working.

[DarrylBurke]

I had to add a setFocusable() so that it could listen the keboard.

Swing is designed to use Key Bindings. Learning how they work and reimplementing without a KeyListener would be time well spent.

db