Java，创建并编辑Panel的Graphics对象？

import javax.swing.JPanel;
import java.awt.Graphics;

public class MyPanel extends JPanel {
    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        // 在这里进行绘图操作
    }
}

import javax.swing.JFrame;
import java.awt.Color;
import java.awt.Graphics;

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

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

        frame.setSize(400, 300);
        frame.setVisible(true);
    }
}

class MyPanel extends JPanel {
    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.setColor(Color.RED);
        g.fillRect(50, 50, 100, 100);
    }
}