Thread: How to draw a rectangle on a JPanel, from a different class?

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

Good first post, and you've created an excellent study to explore how painting in Java works and as the basis from which to ask a decent question. Welcome!

You're close, but the key to remember when painting on a specific component is to remember that whatever is doing the painting must have a reference to that component's graphics object. There are a number of ways to accomplish that, and you can explore those with the example you've created, but one of the simplest ways is to pass the Graphics object to the painting method, like this (some of the code you had is now irrelevant, but I left it as is):

class Surface extends JPanel {
 
	public void paintComponent(Graphics g){
 
		super.paintComponent(g);
 
		DrawRect d = new DrawRect(this);
		d.draw( g );
 
	}
 
}
 
class DrawRect {
 
	Surface surface;
	Graphics g;
 
	public DrawRect(Surface surface)
    {
		g = surface.getGraphics();
	}
 
    public void draw( Graphics g )
    {
        g.fillRect(20,20,100,50); // (now this will work).
    }
 
}

If you haven't already, I recommend you read Performing Custom Painting in the Java tutorials.

It works, thanks But I don't understand why what I did before didn't work.

You said I needed a reference to the Graphics object of Surface. But I did obtain one inside DrawRect class, in the previous version, and used this reference inside the draw() method. The only difference is that with the version you suggested (that works), the method gets the Graphics object by directly passing it into the method, and in my previous version (that didn't work), I used

g = surface.getGraphics();

in the constructor of DrawRect, and the method draw() simply used g. Why does it matter?

It actually is "working," but not persistently. Notice what happens when you run your version and then resize the frame by grabbing the corner and move it around. You can see the rectangle flash in and out like an early moving picture. I think the answer to why your version does not draw persistently is because of the lifespan of the surface's Graphics object. I'm not sure that's addressed specifically in the link I posted earlier, but you may be able to find it through further searching and study into Swing's painting mechanism.