I m assuming you aren t aware of the Android Chronometer - it already has a basic stopwatch function. You need to work with its peculiarities a bit, but it s not hard to get it to do what you want once you understand how it works.

There are a few ways that time is calculated on the phone, but the two main ones are:

1. The "real time", such as right now according to my computer clock, it is 11:23am in England. However, this can change if my computer contacts a time server and is told it has the wrong time, or if I were travelling with a laptop and crossed a timezone boundary. Using this would wreak havoc with your stopwatch as the measured time could change at any time.

2. The "elapsed time since boot", which is the number of milliseconds since the phone was switched on. This number doesn t bear any relation to the real time it is, but it will behave in a perfectly predictable manner. This is what the Android Chronometer uses.

The Chronometer is essentially a count up timer, comparing the current SystemClock.elapsedRealtime() against the elapsedRealtime() that was set fot its base time. The difference between the two, divided by 1000, is the number of seconds since the timer was started. However, if you stop the timer and then start it again, you will get a counter-intuitive result - the timer will show the elapsed time as if it had never stopped. This is because you need to adjust its base time to take into consideration the time it was stopped. This is simple to do:

// When you re stopping the stopwatch, use this
// This is the number of milliseconds the timer was running for
elapsedTimeBeforePause = SystemClock.elapsedRealtime() - timer.getBase();

// When you re starting it again:
timer.setBase(SystemClock.elapsedRealtime() - elapsedTimeBeforePause);

Edit: Here is the full code for a basic stopwatch, which displays your time in a TextView rather than the Chronometer widget declared in your XML file.

public class TestProject extends Activity {
    TextView textGoesHere;
    long startTime;
    long countUp;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        Chronometer stopWatch = (Chronometer) findViewById(R.id.chrono);
        startTime = SystemClock.elapsedRealtime();

        textGoesHere = (TextView) findViewById(R.id.textGoesHere);
        stopWatch.setOnChronometerTickListener(new OnChronometerTickListener(){
            @Override
            public void onChronometerTick(Chronometer arg0) {
                countUp = (SystemClock.elapsedRealtime() - arg0.getBase()) / 1000;
                String asText = (countUp / 60) + ":" + (countUp % 60); 
                textGoesHere.setText(asText);
            }
        });
        stopWatch.start();
    }
}

In your main.xml you need to have this

<Chronometer android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:id="@+id/chrono"
    android:visibility="gone"/>

There s undoubtedly a way to get the Chronometer to work without declaring it in the XML file, but the constructor Chronometer stopwatch = new Chronometer(this); didn t work properly.

The above code displays the elapsed time in a very basic way. For example, if only 5 seconds have gone by, it will show 0:5 rather than the 0:05 you probably want. Fixing that is not hard to do, but I ll leave that for you to work out! :)

How about this one Digital Clock.

But basically you should be able to find many java implementations by googling for keywords like java, stopwatch, digital clock, ...

You could take a look at Stopwatch Applet and modify as needed (Oops no source with this one)