通过Hander进行界面刷新

     Timer timer;
     TimerTask task;
     Handler handler;//先声明这3个变量

 //在onCreate方法内

     handler = new Handler(){

            @Override        

            public void handleMessage(Message msg){

            super.handleMessage(msg);

            Bundle data = msg.getData();

            String num2 = data.getString("num1");

     //     System.out .println("hello");

            txt.setText(num2);//文本显示随机数

            }

        };

         task=new TimerTask() {        

            @Override

            public void run() {

               int num =(int)(Math.random()*300);//随机数

               String num1 = String.valueOf(num);//类型转换

               Message msg = new Message();

               Bundle data = new Bundle();

               data.putString("num1", num1);//存入数据

               msg.setData(data);

               handler.sendMessage(msg);//发送信息

            }

        };

         timer = new Timer();    

        timer.schedule(task, 500, 1000);//半秒后执行task，每秒执行一次
 //在onCreate方法内