How to get any identifier of the topmost activity?

Question

I have a service and its behavior must change when topmost Activity changes. Say, Activity A is active and then service starts some kind of processing. This processing must

Answer 1

Create a baseActivity and extends all other Activities from baseActivity, in onResume save Current Activity in a public static Filed in a Global class (in my case extends Application)

public class baseActivity extends Activity {
    @Override
    protected void onResume() {
        super.onResume();
        Global.Activity = this;
    }
}

Other Activities

    public class mainActivity extends baseActivity {
        // You Can Use Global.Activity Everywhere ! , Even in a Object Extended Class
    }