java编写程序，提示说20行有问题

Button1Handler 是普通类的话，就定义在主类外面。静态内部类的话，就用static修饰。

import java.awt.*;
import java.awt.event.*;
public class WelcomeGUL {
    static Frame f=new Frame("Welcome GUL");
    static Label lb1=new Label("欢迎进入java的图形用户界面!");
    static Label lb2=new Label("在这里显示按钮事件描述");
    static Button b1=new Button("单击我");
    static Button b2=new Button("退出");
    public static void main(String[] args) {
        f.setSize(200,200);
        f.setLayout(new FlowLayout());
        f.add(lb1);
        f.add(lb2);
        f.add(b1);
        f.add(b2);
        b1.addMouseListener(new Button1Handler());
        b2.addActionListener(new Button2Handler());
        f.setVisible(true);
    }
    
}
class Button1Handler implements MouseListener{
    public void mouseClicked(MouseEvent e) {
        WelcomeGUL.lb2.setText("已单击鼠标!");
    }
    public void mouseEntered(MouseEvent e) {
        WelcomeGUL.lb2.setText("已进入按钮上方!");
    }
    public void mouseExited(MouseEvent e) {
        WelcomeGUL.lb2.setText("已离开按钮上方!");
    }
    public void mousePressed(MouseEvent e) {
        WelcomeGUL.lb2.setText("已按下按钮!");
    }
    public void mouseReleased(MouseEvent e) {
    }
}
class Button2Handler implements ActionListener{
    public void actionPerformed(ActionEvent e) {
        System.exit(0);
    }
}

静态内部类

import java.awt.*;
import java.awt.event.*;
public class WelcomeGUL {
    static Frame f=new Frame("Welcome GUL");
    static Label lb1=new Label("欢迎进入java的图形用户界面!");
    static Label lb2=new Label("在这里显示按钮事件描述");
    static Button b1=new Button("单击我");
    static Button b2=new Button("退出");
    public static void main(String[] args) {
        f.setSize(200,200);
        f.setLayout(new FlowLayout());
        f.add(lb1);
        f.add(lb2);
        f.add(b1);
        f.add(b2);
        b1.addMouseListener(new Button1Handler());
        b2.addActionListener(new Button2Handler());
        f.setVisible(true);
    }
    static class Button1Handler implements MouseListener{
        public void mouseClicked(MouseEvent e) {
            WelcomeGUL.lb2.setText("已单击鼠标!");
        }
        public void mouseEntered(MouseEvent e) {
            WelcomeGUL.lb2.setText("已进入按钮上方!");
        }
        public void mouseExited(MouseEvent e) {
            WelcomeGUL.lb2.setText("已离开按钮上方!");
        }
        public void mousePressed(MouseEvent e) {
            WelcomeGUL.lb2.setText("已按下按钮!");
        }
        public void mouseReleased(MouseEvent e) {
        }
    }
    static class Button2Handler implements ActionListener{
        public void actionPerformed(ActionEvent e) {
            System.exit(0);
        }
    }
}