如何在面板中使用两个不同的类进行绘制？

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

class Rectangle {
    public void draw(Graphics g) {
        g.drawRect(50, 50, 100, 50);
    }
}

class Circle {
    public void draw(Graphics g) {
        g.drawOval(150, 150, 50, 50);
    }
}

class DrawingPanel extends JPanel {
    private Rectangle rectangle;
    private Circle circle;

    public DrawingPanel() {
        rectangle = new Rectangle();
        circle = new Circle();
    }

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        rectangle.draw(g);
        circle.draw(g);
    }
}

public class Main {
    public static void main(String[] args) {
        JFrame frame = new JFrame("Drawing Panel");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

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

        frame.setSize(300, 300);
        frame.setVisible(true);
    }
}