Android BroadCastReceiver用于音量键的上下

这是我的意图过滤器

IntentFilter filter = new IntentFilter();
filter.addAction("android.media.VOLUME_CHANGED_ACTION");

和我的onReceive方法是

public void onReceive(Context arg0,Intent intent) {
      KeyEvent ke = (KeyEvent)intent.getExtras().get(Intent.EXTRA_KEY_EVENT);
        if (ke .getKeyCode() == KeyEvent.KEYCODE_VOLUME_Up) {
                System.out.println("I got volume up event");
         }else if (ke .getKeyCode() == KeyEvent.KEYCODE_VOLUME_DOWN) {
                System.out.println("I got volume key down event");
         }
    }

它给我一个空KeyEvent …任何想法？