[Eric]

What you want to paint on, is not the JFrame, but on its contentPane.

This is more or less the way I always do it, by using JPanels:

Code:

import javax.swing.JFrame;

class{
    main{
         //create JFrame
         JFrame frame = new JFrame ("A JFrame");
         frame.setDefaultCloseOperation (JFrame.EXIT_ON_CLOSE);
        
         //create instance of drawing class
         //i guess you could do it on init()..
         Design d = new Design();
        
         //add contentPane to frame
         //add drawings to contentPane
         frame.getContentPane().add(d);
         frame.pack();
         frame.setVisible(true);

    }
}

Code:

import javax.swing.JPanel;

Design class extends JPanel{
    public Design(){ //constructor
         //add initial values
         ...
         //set panel color and size
         setBackground (Color.gray);
         setPreferredSize (new Dimension(600,600));
    }    

    ...stuff
    public void paint(Graphics g) {
        //drawings go here
        super.paint(g);
        ....stuff
    }
    ...stuff
}

Greetings

Eric