一个Java问题

Java code:

import java.awt.*;
import java.awt.event.*;

public class TaskList extends Frame implements ActionListener
{
private Button b1 = new Button("确定");
private Button b2 = new Button("删除");
private Button b3 = new Button("增加优先级");
private Button b4 = new Button("降低优先级");
private TextField text = new TextField(15);

private List list = new List();
private Label task_label = new Label("工作事项");
private Label priority_label = new Label("改变事件优先级");
private class WindowCloser extends WindowAdapter
{
public void windowClosing(WindowEvent e){
dispose();
System.exit(0);
}
}
public void setup(){
Panel p = new Panel(new FlowLayout());
p.add(b1);
p.add(b2);
Panel q = new Panel(new FlowLayout());
q.add(b3);
q.add(priority_label);
q.add(b4);
Panel r = new Panel(new FlowLayout());
r.add(task_label);
r.add(text);
setLayout(new GridLayout(4,1));
add(r);
add(q);
add(list);
add(p);
}
public TaskList(){
super("工作事项表");
this.setup();
b1.addActionListener(this);
b2.addActionListener(this);