android 倒计时

public class MainActivity extends AppCompatActivity {

    private TextView djs;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        djs=(TextView)findViewById(R.id.djs);

        new CountDownTimer(30000,1000){
            public void onTick(long millisUntilFinished) {

                djs.setText("seconds remaining: " + millisUntilFinished / 1000);
            }

            public void onFinish() {
                djs.setText("done!");

            }
        }.start();

    }
}