public class Soma extends AppCompatActivity {
TextView  tempo;
private  int counter= 45;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_soma);
tempo = (TextView) findViewById(R.id.tempo);
  new CountDownTimer(45000, 1000) {
        @Override

        public void onTick(long millisUntilFinished) {
         tempo.setText(String.valueOf(counter));
            counter--;
        }

        @Override
        public void onFinish() {
            tempo.setText("Acabou o tempo!");
        }
    };