这么简单的程序哪里错了？

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);         
         g.drawRect(20,20,20+(int)(Math.random()*100),20+(int)(Math.random()*100));                
    }
}