Create an inner Handler class for MainActivity, and create a new Messenger field using this class.

// some methods omitted
public class MainActivity extends Activity {
    Boolean isBound = false;
    Messenger mServiceMessenger;

    ServiceConnection mServiceConnection = new ServiceConnection() {
      @Override  
      public void onServiceConnected(ComponentName name, IBinder binder) {
        isBound = true;
        mServiceMessenger = new Messenger(binder);
      }

      @Override
      public void onServiceDisconnected(ComponentName name) {
        isBound = false;
      }
    }; 


  class mActivityHandler extends Handler { 
       MainActivity mmainActivity;
    mActivityHandler(MainActivity mainActivity) {
  mmainActivity = mainActivity; 
   }
     @Override
      public void handleMessage(Message msg) {

    } 
  }
}

// some methods omitted
public class GeolocationService extends Service {
    int mLatitude;
    int mLongitude;
    Messenger mMessenger = new Messenger(new LocationHandler());

    @Override
    public IBinder onBind(Intent intent) {
      return mMessenger.getBinder();
    }

   class LocationHandler extends Handler {
      @Override
      public void handleMessage(Message msg) {
        mLatitude = getLatitude();
        mLongitude = getLongitude();
      }
    }
}

  public class Difficulty {
    public static Boolean HARDMODE = false;    
  }