Drawing an object using getGraphics() without extending JFrame

How can I draw an object without a class (which extends JFrame)? I found getGraphics method but it doesnt draw the object.

import javax.swing.*;
import java.awt.*;
public class Main {
    public static void main(String[] args) {
        JFrame frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
        frame.setSize(600, 400);

        JPanel panel = new JPanel();
        frame.add(panel);

        Graphics g = panel.getGraphics();
        g.setColor(Color.BLUE);
        g.fillRect(0, 0, 100, 100);
    }
}