Cara Membuat Aplikasi Stopwatch berbasis GUI di Java

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

public class Stopwatch extends JFrame implements ActionListener {
private int jam = 0;
private int menit = 0;
private int detik = 0;
private int delay = 1000;
private Timer timer = null;
private Label label = new Label ("00:00:00");
private Button btnStart = new Button ("Start");
private Button btnPause = new Button ("Pause");
private Panel panel = new Panel();

public Stopwatch(){
super("Stopwatch created by Randi Heri & Syahrul");
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
setLocation(100,100);
setSize(500,300);
btnStart.addActionListener(this);
btnPause.addActionListener(this);
panel.setLayout(new GridLayout(1,2));
panel.add(btnStart);
panel.add(btnPause);
label.setFont(new Font("arial",Font.BOLD,25));
label.setAlignment(Label.CENTER);
getContentPane().setLayout(new BorderLayout());
getContentPane().add(label,BorderLayout.CENTER);
getContentPane().add(panel,BorderLayout.SOUTH);
setVisible(true);
timer = new Timer(delay, display);}

public void actionPerformed(ActionEvent ae){
if (ae.getSource() == btnStart)
{
if(!timer.isRunning())
timer.start();
}
else
{
if (timer.isRunning())
timer.stop();
}
}

private ActionListener display = new ActionListener() {
public void actionPerformed(ActionEvent e){
String str1 = Integer.toString(jam);
String str2 = Integer.toString(menit);
String str3 = Integer.toString(detik);

if (jam < 10) str1 = "0" + str1;
if (menit < 10) str2 = "0" + str2;
if (detik < 10) str3 = "0" + str3;

label.setText(str1 + ":" + str2 + ":" + str3);
detik++;
if(detik == 60) {
detik = 0;
menit++;
if (menit == 60) {
menit = 0;
jam++;
if (jam == 24)
jam = 0;
}
}
}
};
public static void main (String[]args) {
new Stopwatch();
}
}