FATAL EXCEPTION: main
         Process: com.example.zhongzhi.gate_control_scheme, PID: 2472
         java.lang.IllegalStateException: Timer already cancelled.
           at java.util.Timer.sched(Timer.java:397)
           at java.util.Timer.schedule(Timer.java:248)
           at com.example.zhongzhi.gate_control_scheme.MainActivity.onClick(MainActivity.java:401)
           at android.view.View.performClick(View.java:5637)
           at android.view.View$PerformClick.run(View.java:22429)
           at android.os.Handler.handleCallback(Handler.java:751)
           at android.os.Handler.dispatchMessage(Handler.java:95)
           at android.os.Looper.loop(Looper.java:154)
           at android.app.ActivityThread.main(ActivityThread.java:6119)
           at java.lang.reflect.Method.invoke(Native Method)
           at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
           at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)

mTimer.cancel();
mTimer = null;
mTimerTask.cancel();
mTimerTask = null;

public class MainActivity extends AppCompatActivity {

  private static String TAG = "TimerDemo";
  private TextView mTextView = null;
  private Button mButton_start = null;
  private Button mButton_pause = null;
  private Timer mTimer = null;
  private TimerTask mTimerTask = null;
  private Handler mHandler = null;
  private static int count = 0;
  private boolean isPause = false;
  private boolean isStop = true;
  private static int delay = 1000; //1s
  private static int period = 1000; //1s
  private static final int UPDATE_TEXTVIEW = 0;

  @Override
  public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    mTextView = (TextView)findViewById(R.id.mytextview);
    mButton_start = (Button)findViewById(R.id.mybutton_start);
    mButton_pause = (Button)findViewById(R.id.mybutton_pause);


    mButton_start.setOnClickListener(new Button.OnClickListener() {
      public void onClick(View v) {
        if (isStop) {
          Log.i(TAG, "Start");
        } else {
          Log.i(TAG, "Stop");
        }

        isStop = !isStop;

        if (!isStop) {
          startTimer();
        }else {
          stopTimer();
        }

        if (isStop) {
          mButton_start.setText(R.string.start);
        } else {
          mButton_start.setText(R.string.stop);
        }
      }
    });

    mButton_pause.setOnClickListener(new Button.OnClickListener() {
      public void onClick(View v) {
        if (isPause) {
          Log.i(TAG, "Resume");
        } else {
          Log.i(TAG, "Pause");
        }

        isPause = !isPause;

        if (isPause) {
          mButton_pause.setText(R.string.resume);
        } else {
          mButton_pause.setText(R.string.pause);
        }
      }
    });

    mHandler = new Handler(){
      @Override
      public void handleMessage(Message msg) {
        switch (msg.what) {
          case UPDATE_TEXTVIEW:
            updateTextView();
            break;
          default:
            break;
        }
      }
    };
  }

  private void updateTextView(){
    mTextView.setText(String.valueOf(count));
  }

  private void startTimer(){
    if (mTimer == null) {
      mTimer = new Timer();
    }

    if (mTimerTask == null) {
      mTimerTask = new TimerTask() {
        @Override
        public void run() {
          Log.i(TAG, "count: "+String.valueOf(count));
          sendMessage(UPDATE_TEXTVIEW);

          do {
            try {
              Log.i(TAG, "sleep(1000)...");
              Thread.sleep(1000);
            } catch (InterruptedException e) {
            }
          } while (isPause);

          count ++;
        }
      };
    }

    if(mTimer != null && mTimerTask != null )
      mTimer.schedule(mTimerTask, delay, period);

  }

  private void stopTimer(){
    if (mTimer != null) {
      mTimer.cancel();
      mTimer = null;
    }
    if (mTimerTask != null) {
      mTimerTask.cancel();
      mTimerTask = null;
    }
    count = 0;
  }

  public void sendMessage(int id){
    if (mHandler != null) {
      Message message = Message.obtain(mHandler, id);
      mHandler.sendMessage(message);
    }
  }
}

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
  android:layout_width="fill_parent"
  android:layout_height="fill_parent"
  android:orientation="vertical" >
  <TextView
    android:id="@+id/mytextview"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:gravity="center"
    android:text="@string/number" />

  <LinearLayout
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:gravity="center"
    android:orientation="horizontal" >

    <Button
      android:id="@+id/mybutton_start"
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:text="@string/start" />

    <Button
      android:id="@+id/mybutton_pause"
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:text="@string/pause" />
  </LinearLayout>
</LinearLayout>

<?xml version="1.0" encoding="utf-8"?>
<resources>
  <string name="app_name">TimerDemo</string>
  <string name="number">0</string>
  <string name="start">start</string>
  <string name="stop">stop</string>
  <string name="pause">pause</string>
  <string name="resume">resume</string>
</resources>