关于timer和timertask的有关问题

public class mainActivity extends Activity {    Button button;    Button button2;    @Override    public void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.main);        button = (Button) findViewById(R.id.btn1);        button2 = (Button) findViewById(R.id.btn2);        button.setOnClickListener(new OnClickListener() {            @Override            public void onClick(View v) {                timer = new Timer();                timer.schedule(task, 1000, 1000);            }        });        button2.setOnClickListener(new OnClickListener() {            @Override            public void onClick(View v) {                这里该怎么写让他完全停止 再次点击开始的时候不报错            }        });    }    Timer timer;    TimerTask task = new TimerTask() {        @Override        public void run() {            System.out.println("a");        }    };}