best:using_a_factory_jpanel [SwingWiki]

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This is a way to override the paint method using a factory and an anonymous class. This is opposed to creating a separate, named class that extends JFrame.

import javax.swing.*;
import java.awt.*;
 
public class Test {
    public static void main(String[] args) {
        new Test().run();
    }
    public void run() {
        JFrame frame = FrameFactory.getInstance();
        frame.setBounds(50, 50, 300, 300);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
    }
}
class FrameFactory {
    private FrameFactory () {}
    public static JFrame getInstance() {
        return new JFrame() {
            public void paint(Graphics graphics) {
                Graphics2D g = (Graphics2D) graphics;
                Dimension size = getSize();
                g.clearRect(0, 0, (int)size.getWidth(), (int)size.getHeight());
                g.drawLine(0, 0, (int)size.getWidth(), (int)size.getHeight());
            }
        };
    }
}