How would I draw on a JPanel to then be shown on a JFrame?

Question

I've tried to research how Java's 2D rendering works, but I could never understand it. Here is the code in my main class:

import java.awt.Dimension;

import javax.swing.JFrame;
import javax.swing.JPanel;

public class Main{

    public static void main(String args[]) {
        JFrame frame = new JFrame();
        frame.setSize(new Dimension(500,500));
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setTitle("JFrame testing");
        frame.setVisible(true);
        Frame panel = new Frame();
        frame.add(panel);
    }
}

And then here is for the JPanel class:

import java.awt.Graphics;

import javax.swing.JPanel;

public class Frame extends JPanel{
    private static final long serialVersionUID = 1L;

    public Frame() {
        Graphics g = this.getGraphics();
        g.drawRect(0, 0, 100, 100);
        this.paintComponent(g);
    }
}

I am also getting this exception, but I'm not sure what it means:

Exception in thread "main" java.lang.NullPointerException
    at Frame.<init>(Frame.java:10)
    at Main.main(Main.java:18)

I'm basically just trying to draw a rectangle onto a panel to be shown on the frame I've created. I've heard about the paintComponent method, but I also don't fully understand that.

John Joe · Accepted Answer · 2018-02-13T03:59:31.003

1

You should Never use getGraphics() of a Component.

Try below code

 public void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.drawRect(0, 0, 100, 100);
        //this.paintComponent(g);
    }

Edit

"why is super.paintComponent(g); called again inside the method?"

The documentation of paintComponent says it pretty well:

if you do not invoker super's implementation you must honor the opaque property, that is if this component is opaque, you must completely fill in the background in a non-opaque color. If you do not honor the opaque property you will likely see visual artifacts.

edited Feb 13 '18 at 03:59

answered Feb 13 '18 at 03:37

John Joe

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Thank you, but do you mind explaining what this does because I'm not really sure what the paintComponent method does. – Walker Feb 13 '18 at 03:42
when your project running, `paintComponent` function will be called by passing in the Graphics parameter as a graphics context onto which you can draw. – John Joe Feb 13 '18 at 03:46
So I learned that paintComponent is overriding a Jcomponent object, but why is `super.paintComponent(g);` called again inside the method? – Walker Feb 13 '18 at 03:50
You may go through the [tutorial](https://docs.oracle.com/javase/tutorial/uiswing/painting/index.html) – John Joe Feb 13 '18 at 03:54
Sorry I just updated my comment to: So I learned that paintComponent is overriding a Jcomponent object, but why is `super.paintComponent(g);` called again inside the method? I just tried without that and it still worked, so what's the point in including it? – Walker Feb 13 '18 at 03:55
Ok, thanks for all your help, it really helped me out. – Walker Feb 13 '18 at 04:02
you can find more answer here https://stackoverflow.com/questions/28724609/what-does-super-paintcomponentg-do – John Joe Feb 13 '18 at 04:03

How would I draw on a JPanel to then be shown on a JFrame?

1 Answers1