public class MainActivity extends AppCompatActivity {  private int minute = 30;//這是分鐘  private int second = 0;//這是分鐘后面的秒數。這里是以30分鐘為例的，所以，minute是30，second是0  private TextView timeView;  private Timer timer;  private TimerTask timerTask;  //這是接收回來處理的消息  private Handler handler = new Handler() {    public void handleMessage(Message msg) {      if (minute == 0) {        if (second == 0) {          timeView.setText("Time out !");          if (timer != null) {            timer.cancel();            timer = null;          }          if (timerTask != null) {            timerTask = null;          }        } else {          second--;          if (second >= 10) {            timeView.setText("0" + minute + ":" + second);          } else {            timeView.setText("0" + minute + ":0" + second);          }        }      } else {        if (second == 0) {          second = 59;          minute--;          if (minute >= 10) {            timeView.setText(minute + ":" + second);          } else {            timeView.setText("0" + minute + ":" + second);          }        } else {          second--;          if (second >= 10) {            if (minute >= 10) {              timeView.setText(minute + ":" + second);            } else {              timeView.setText("0" + minute + ":" + second);            }          } else {            if (minute >= 10) {              timeView.setText(minute + ":0" + second);            } else {              timeView.setText("0" + minute + ":0" + second);            }          }        }      }    }  };  @Override  protected void onCreate(Bundle savedInstanceState) {    requestWindowFeature(Window.FEATURE_NO_TITLE);    super.onCreate(savedInstanceState);    setContentView(R.layout.activity_main);    timeView = (TextView) findViewById(R.id.tv);    timeView.setText(minute + ":" + second);    timerTask = new TimerTask() {      @Override      public void run() {        Message msg = new Message();        msg.what = 0;        handler.sendMessage(msg);      }    };    timer = new Timer();    timer.schedule(timerTask, 0, 1000);  }  @Override  protected void onDestroy() {    if (timer != null) {      timer.cancel();      timer = null;    }    if (timerTask != null) {      timerTask = null;    }    minute = -1;    second = -1;    super.onDestroy();  }  @Override  protected void onStart() {    super.onStart();  }  @Override  protected void onStop() {    super.onStop();  }  @Override  protected void onResume() {    super.onResume();  }  @Override  protected void onRestart() {    super.onRestart();  }  @Override  protected void onPause() {    super.onPause();  }}