Simple Java Clock app for testing the Observer pattern.

ButtonAction.java

// PACKAGE GOES HERE <--------

import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

public class ButtonAction implements ActionListener {

	private Clock clock = new Clock();
	@Override
	public void actionPerformed(ActionEvent e) {
		if (clock.timer.isRunning()){
			clock.stopClock();
		} else {
			clock.restartClock();
		}
	}	
}

Clock.java

// PACKAGE GOES HERE <---------

import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.GregorianCalendar;
import java.util.Observable;

import javax.swing.Timer;

public class Clock extends Observable implements ActionListener {

	public static Timer timer;   

	public Clock(){
		this.startClock();
	}
	
	public String getTime(){
		String time = "";

		GregorianCalendar calendario = new GregorianCalendar();   
		int h = calendario.get(GregorianCalendar.HOUR_OF_DAY);   
		int m = calendario.get(GregorianCalendar.MINUTE);   
		int s = calendario.get(GregorianCalendar.SECOND);   
		time += ((h < 10) ? "0" : "") + h + ":";
		time += ((m < 10) ? "0" : "") + m + ":";
		time += ((s < 10) ? "0" : "") + s;

		return time;
	}
	
	public void actionPerformed(ActionEvent e) {
		setChanged();
		notifyObservers(this.getTime());
	}
	
	public void startClock() {
		if (timer == null) {
			timer = new javax.swing.Timer(1000, this);
			timer.setInitialDelay(0);
			timer.start();
		}
	}

	public void restartClock(){
		if (!timer.isRunning()){
			timer.restart();	
		}
	}
		
	public void stopClock() {
		if (timer != null) {
			timer.stop();
		}
	}
}

Form.java

// PACKAGE GOES HERE <------------
import java.awt.FlowLayout;
import java.awt.Font;
import java.util.Observable;
import java.util.Observer;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;

public class Form implements Observer {

	private Clock clock = new Clock();
	private JLabel label = new JLabel();
	private JButton button = new JButton("Start/Stop");
	private static JFrame frame = new JFrame("Clock :D");

	public Form(){
		this.mountDisplay();
		clock.addObserver(this);
	}

	public static void main(String[] args){
		javax.swing.SwingUtilities.invokeLater(new Runnable() {  
			@Override
			public void run() {  
				Form main = new Form();
			}
		});   
	}

	public void mountDisplay(){  
		JPanel panel = new JPanel();   

		panel.add(label);
		panel.add(button);
		panel.setLayout(new FlowLayout(FlowLayout.CENTER));

		label.setFont(new Font("Arial", Font.ITALIC, 25));
		frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);   

		frame.getContentPane().add(panel);   

		frame.setResizable(false);   
		frame.setBounds(200, 200, 500, 110);
		frame.setLocationRelativeTo(null);  
		frame.setVisible(true); 
		button.addActionListener(new ButtonAction());
		label.setText(clock.getTime());

	}   

	@Override
	public void update(Observable arg0, Object arg1) {
		System.out.println("Hora: " + arg1);
		label.setText((String)arg1);
	}
}