Drawing to a JPanel-type class, from a different class.

[AvivC]

Hello everyone, now to this forum :)

I'm fairly new to 2D graphics in Java.
I learned how to draw simple shapes on a JPanel, using the paintComponent method.

Now I'm trying to do the same thing, except the drawing should be done from an exterior class, to the class that extends JPanel.

I tried something that I believe should work, but it doesn't. Could you explain to me the standard way to do such a thing?

Here is my attempt:

Class Main:

package m;

import java.util.*;
import java.awt.*;
import java.awt.Event.*;
import javax.swing.*;

public class Main extends JFrame {

	public static void main(String[] args) {

		Main m = new Main();
		
	}
	
	public Main(){
		
		setDefaultCloseOperation(EXIT_ON_CLOSE);
		setTitle("Bla");
		setSize(500,500);
		Surface s = new Surface();
		add(s);
		setVisible(true);
		
	}

}

Class Surface:

package m;

import java.util.*;
import java.awt.*;
import java.awt.Event.*;
import javax.swing.*;

public class Surface extends JPanel {

	public void paintComponent(Graphics g){
		
		super.paintComponent(g);
		
		DrawRect d = new DrawRect(this);
		d.draw();
		
	}
	
}

Class DrawRect:

package m;

import java.util.*;
import java.awt.*;
import java.awt.Event.*;
import javax.swing.*;

public class DrawRect {

	Surface surface;
	Graphics g;
	
	public DrawRect(Surface surface){
		this.surface = surface;
		g = surface.getGraphics();
	}
	
    public void draw(){
    g.fillRect(20,20,100,50); // (this won't work).
    }

}

Thanks a lot :)

[Norm]

Also posted at How to draw a rectangle on a JPanel, from a different class?