TimerExample.java

package timerexample;

import java.awt.BorderLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.Observable;
import java.util.Observer;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.SwingUtilities;
import javax.swing.Timer;

public class TimerExample extends JFrame implements Observer, ActionListener {

    TimerModel m;
    long lastTime;
    
    JLabel time;
    JButton start, stop;

    TimerExample() {
        m = new TimerModel();
        this.setLayout(new BorderLayout());
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        time = new JLabel();
        String str = String.format("%02d:%02d", m.getTime() / 60, m.getTime() % 60);
        time.setText(str);
        time.setHorizontalAlignment(JLabel.CENTER);
        time.setVerticalAlignment(JLabel.CENTER);
        this.add(time, BorderLayout.CENTER);
        start = new JButton("Start");
        start.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                m.start();
            }
        });
        this.add(start, BorderLayout.WEST);
        stop = new JButton("Stop");
        stop.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                m.stop();
            }
        });
        this.add(stop, BorderLayout.EAST);
        lastTime = System.nanoTime();
        Timer t = new Timer(20, this);
        t.start();
    }

    @Override
    public void setVisible(boolean b) {
        super.setVisible(b);
        //Observer reference added here to avoid reference leak in contructor
        m.addObserver(this);
    }

    public static void main(String[] args) {
        TimerExample x = new TimerExample();
        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                x.setSize(300, 200);
                x.setVisible(true);
            }

        });
    }

    @Override
    public void update(Observable o, Object arg) {
        String str = String.format("%02d:%02d", m.getTime() / 60, m.getTime() % 60);
        time.setText(str);
    }

    @Override
    public void actionPerformed(ActionEvent e) {
        long actual = System.nanoTime();
        double diff = actual - this.lastTime - 1e9;
        if (diff > -2.5e7) {
            this.lastTime += 1e9;
            System.out.println(actual + ",");
            m.tic();
        }
    }

    

}

TimerModel.java

package timerexample;

import java.util.Observable;

public class TimerModel extends Observable {

    int time = 600;
    State s = new StatePause();

    public void decrease() {
        time--;
        setChanged();
        notifyObservers();
    }

    public int getTime() {
        return time;
    }
    
    public void tic(){
        s.tic();
    }

    public void start() {
        s = new StateStart();
    }

    public void stop() {
        s = new StatePause();
    }

    interface State {

        void tic();
    }

    class StateStart implements State {

        @Override
        public void tic() {
            decrease();
        }
    }

    class StatePause implements State {

        @Override
        public void tic() {
        }
    }

}