这么简单的程序哪里错了？
import java.awt.*;
import javax.swing.*;
import java.awt.event.*;

class DrawRectangle extends JFrame
{
    public static void main(String[] args)
    {
        DrawRectangle dr=new DrawRectangle();
        dr.setSize(RECT_WIDTH,RECT_HEIGHT);
        
        Toolkit kit=Toolkit.getDefaultToolkit();
        Dimension screenSize=kit.getScreenSize();

        dr.setLocation((screenSize.width-RECT_WIDTH)/2,(screenSize.height-RECT_HEIGHT)/2);
        
        dr.setDefaultCloseOperation(dr.EXIT_ON_CLOSE);
        
        dr.add(new MyPanel());
        dr.setVisible(true);
           
    }
    public static final int RECT_WIDTH=800;
    public static final int RECT_HEIGHT=600;
}

class MyPanel extends Panel
{
    Button button=new Button("刷新");
    add(button);
    button.addActionListener(new 
       ActionListener()
       {
           public void actionPerformed(ActionEvent event)
           {
               repaint();
           }
       });

    public void paint(Graphics g)
    {
                           
         g.setColor(Color.RED);
         
         g.drawRect(20,20,20+(int)(Math.Random()*100),20+(int)(Math.Random()*100));
                
    }
}
------解决方案--------------------

import java.awt.*;

import javax.swing.*;

import java.awt.event.*;



public class Test extends JFrame {

    public static void main(String[] args) {

        Test dr = new Test();

        dr.setSize(RECT_WIDTH,RECT_HEIGHT);        

        Toolkit kit = Toolkit.getDefaultToolkit();

        Dimension screenSize = kit.getScreenSize();



        dr.setLocation((screenSize.width-RECT_WIDTH)/2, (screenSize.height-RECT_HEIGHT)/2);        

        dr.setDefaultCloseOperation(dr.EXIT_ON_CLOSE);



        dr.add(new MyPanel());

        dr.setVisible(true);

    }

    public static final int RECT_WIDTH = 800;

    public static final int RECT_HEIGHT = 600;

}



class MyPanel extends JPanel {

	public MyPanel(){

		JButton button = new JButton("刷新");

		add(button);

		button.addActionListener(new ActionListener() {

			public void actionPerformed(ActionEvent event){

				repaint();

			}

		});

	}



    public void paint(Graphics g){                           

         g.setColor(Color.RED);