CountDownTimer is an abstract class available in android. This class contains two abstract method i.e. onTick(long) and onFinish().

onTick(long millisUntilFinished): Callback fired on regular interval.
onFinish()                      : Callback fired when the time is up.

This class schedule a countdown until a time in the future, with regular notifications on intervals along the way.
An Example of CountDownTimer class showing a 30 second countdown in a text field:-
Declare a two long constant:-

private final long TOTAL_DURATION = 30000, COUNT_INTERVAL = 1000;

Now in the onCreate() method:-

 new CountDownTimer(TOTAL_DURATION, COUNT_INTERVAL) {

        public void onTick(long millisUntilFinished) {
             timerTextView.setText("seconds remaining: " + millisUntilFinished / 1000);
        }

        public void onFinish() {
              timerTextView.setText("done!");
         }
  }.start();

Your 30 seconds CountDown is ready.