Как разместить квадрат по середине окна

import javax.swing.*;
 
public class GUI extends JFrame {
    public static final int WIDTH = 500;
    public static final int HEIGHT= 400;
 
    public GUI(String s) {
        setTitle(s);
        setSize(WIDTH, HEIGHT);
        MyPanel panel = new MyPanel();
        add(panel);
    }
}

import javax.swing.*;
import java.awt.*;
import java.awt.geom.Rectangle2D;
 
public class MyPanel extends JPanel {
 
    public static final double x1 = 100;
    public static final double WIDTH = (GUI.WIDTH - x1) - x1;
    public static final double y1 = 100;
    public static final double HEIGHT = (GUI.HEIGHT - y1)-y1;
 
    public MyPanel() {
 
    }
 
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        Graphics2D g2 = (Graphics2D) g;
        Rectangle2D rect = new Rectangle2D.Double(x1,y1,WIDTH,HEIGHT);
        g2.setColor(Color.GREEN);
        g2.fill(rect);
    }
}

import javax.swing.*;
import java.awt.*;
import java.awt.geom.Rectangle2D;
 
public class MyPanel extends JPanel {
 
    public static final int WIDTH2 = 200;
    public static final int HEIGHT2 = 200;
 
    public MyPanel() {
 
    }
 
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        Graphics2D g2 = (Graphics2D) g;
        Rectangle2D rect = new Rectangle2D.Double((GUI.WIDTH - WIDTH2) / 2, (GUI.HEIGHT - HEIGHT2) / 2, WIDTH2, HEIGHT2);
        g2.setColor(Color.GREEN);
        g2.fill(rect);
    }
}

    class MyPanel extends JPanel {
 
        public static final int WIDTH = 300;
        public static final int HEIGHT = 200;
 
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            g.setColor(Color.GREEN);
            int leftMargin = (getWidth() - WIDTH) / 2;
            int topMargin = (getHeight() - HEIGHT) / 2;
            g.fillRect(leftMargin, topMargin, WIDTH, HEIGHT);
        }
    }