How to make my own graphic methods to be drawn on jpanel

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Java

I'm creating function which draws a rect on the screen.


The code is below in What I have tried

My Error

Exception in thread "main" java.lang.NullPointerException: Cannot invoke "java.awt.Graphics.fillRect(int, int, int, int)" because the return value of "javax.swing.JPanel.getGraphics()" is null
at myCanvas.fillRect(myCanvas.java:57)
at Main.main(Main.java:15)


For some reason the `super.getGraphics()` is returning null, and I'm unable to fix it.

Similar type of usage of graphics like this, I saw in this [website][1] in example program 2.

How should I fix this error?

What I have tried:

public class myCanvas extends JPanel{
    // some code
    public void fillRect(int w, int h){
        super.getGraphics().fillRect(penX,penY, w, h);
    }
    // some code
}