Create a simple timer in a game

I would be very grateful if someone could explain how to create a simple timer in a game which starts off at 30 seconds and then goes back until 0. At 0 I would then stop the game and invoke a method which would allow the next player to take a turn.

Any advice would be appreciated. Thanks

Edit:

import android.os.CountDownTimer;
import android.widget.TextView;

public class MyCount extends CountDownTimer {
    static TextView timeDisplay;

    public MyCount(long millisInFuture, long countDownInterval) {
        super(millisInFuture, countDownInterval);
    }

    public void onFinish() {
        timeDisplay.setText("Done!");
    }

    public void onTick(long millisUntilFinished) {
        timeDisplay.setText("Left: " + millisUntilFinished / 1000);
    }
}

In a seperate class I have created a method:

public void setTimer() { 
    timeDisplay = new TextView(this);
    this.setContentView(timeDisplay);
    MyCount counter = new MyCount(3开发者_如何学Python0000, 1000);
    counter.start();
}

Within an onClick method I have called the method above:

public void onClick(View v) {
    setTimer();
}

I'd appreciate any advice on this.