Example

import java.awt.*;
import javax.swing.*;

public class Rectangle extends JPanel {
    JFrame frame;

    public void fireUpScreen() {
        frame = new JFrame();
        frame.setVisible(true);
        frame.setSize(600,600);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.add(this);
    }

    public void paintComponent(Graphics g) {
        g.setColor(Color.BLUE);
        g.fillRect(0,0,200,100);
    }

    public static void main(String[] args) {
        Rectangle go = new Rectangle();
        go.fireUpScreen();
    }
}