ActivityForService

public class MainActivity extends AppCompatActivity {

    private String TAG = "MainActivity";
    private TextView timer;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        timer = (TextView) findViewById(R.id.timer);
        startService(new Intent(this, TimerService.class));
        Log.i(TAG, "Started service");
    }

    private BroadcastReceiver br = new BroadcastReceiver() {
        @Override
        public void onReceive(Context context, Intent intent) {
            updateGUI(intent); // or whatever method used to update your GUI fields
        }
    };

    @Override
    public void onResume() {
        super.onResume();
        registerReceiver(br, new IntentFilter(TimerService.COUNTDOWN_BR));
        Log.i(TAG, "Registered broacast receiver");
    }

    @Override
    public void onPause() {
        super.onPause();
        unregisterReceiver(br);
        Log.i(TAG, "Unregistered broacast receiver");
    }

    @Override
    public void onStop() {
        try {
            unregisterReceiver(br);
        } catch (Exception e) {
            // Receiver was probably already stopped in onPause()
        }
        super.onStop();
    }

    @Override
    public void onDestroy() {
        stopService(new Intent(this, TimerService.class));
        Log.i(TAG, "Stopped service");
        super.onDestroy();
    }

    private void updateGUI(Intent intent) {
        if (intent.getExtras() != null) {
            long millisUntilFinished = intent.getLongExtra("countdown", 0);
            timer.setText(String.valueOf((int)millisUntilFinished/1000));
            Log.i(TAG, "Countdown seconds remaining: " + millisUntilFinished / 1000);
        }
    }
}