如何在Java(JFrame)中绘制一个简单的类对象？

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

public class SimpleObjectDrawing extends JFrame {
    
    public SimpleObjectDrawing() {
        setTitle("Simple Object Drawing");
        setSize(400, 400);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setLocationRelativeTo(null);
        setVisible(true);
    }
    
    @Override
    public void paint(Graphics g) {
        super.paint(g);
        
        // 绘制一个简单的类对象
        g.setColor(Color.RED);
        g.fillRect(100, 100, 200, 200);
    }
    
    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> {
            new SimpleObjectDrawing();
        });
    }
}