androidの当前时间动态显示

public class MainActivity extends Activity {
private TextView textview;
private Handler handler;

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
textview = (TextView) findViewById(R.id.textview2);
handler = new Handler() {
public void handleMessage(Message msg) {
textview.setText((String) msg.obj);
}
};
Threads thread = new Threads();
thread.start();
}

class Threads extends Thread {
@Override
public void run() {
try {
while (true) {
SimpleDateFormat sdf = new SimpleDateFormat(
"yyyy年MM月dd日   HH:mm:ss");
String str = sdf.format(new Date());
handler.sendMessage(handler.obtainMessage(100, str));
Thread.sleep(1000);
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}