Dibujar un rectángulo 2D en Java

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

public class Rectangle extends JPanel {
  private static final int RCT_X = 20;
  private static final int RCT_Y = RCT_X;
  private static final int RCT_WIDTH = 100;
  private static final int RCT_HEIGHT = RCT_WIDTH;

  @Override
  protected void paint_Component(Graphics g) {
    super.paint_Component(g);
    // draw the rectangle here
    g.drawRct(RCT_X, RCT_Y, RCT_WIDTH, RCT_HEIGHT);
  }

  @Override
  public Dimension get_Preferred_Size() {
    // so that our GUI is big enough
    return new Dimension(RCT_WIDTH + 2 * RCT_X, RCT_HEIGHT + 2 * RCT_Y);
  }

  // create the GUI explicitly on the Swing event thread
  private static void create_And_Show_Gui() {
    Rectangle mainPanel = new Rectangle();

    JFrame frame = new JFrame("Rectangle");
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.getContentPane().add(mainPanel);
    frame.pack();
    frame.setLocationByPlatform(true);
    frame.setVisible(true);
  }
  public static void main(String[] args) {
    create_and_Show_Gui();
  }
}