在JPanel上绘制图形后，改变窗口大小，图形就消失了，如何解决？

[wuyan19831013]

import javax.swing.*;
import java.awt.*;
public class Main {
JPanel p;
JFrame f;
drawonpanel pp;
public Main() {
p = new JPanel();
f = new JFrame();
f.getContentPane().add(p,BorderLayout.CENTER);
f.setBounds(300,300,400,400);
f.setVisible(true);
f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

// f.pack();
f.show();
pp = new drawonpanel(p);
new Thread(pp).start();
}

public static void main(String args[]){
new Main();
}
}


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

public class drawonpanel implements Runnable{
Graphics g;
int i=0;
public drawonpanel(JPanel p) {
g = p.getGraphics();
}
public void run(){
while(true){
try{
Thread.sleep(100);
}catch(Exception e){}
g.drawString("i am a boy!", 50 + i, 50 + i);
i+=20;
}
//g.drawLine(20,30,100,20);
}
}

如何解决呢？

[NewStarSE]

public class MyCanvas extends JPanel{ //继承并扩展JPanel类
public void paintComponent(Graphics g){//当控件发生变化时，会调用此方法
g.setColor(Color.red); //所以需要重载此方法
g.drawArc(10,10,100,100,0,360);
g.drawRect(50,50,100,100);
g.drawString("测试",30,30);
}
}
当你用到JPanel的地方就用MyCanvas代替