Android : I need to tell an Activity to refresh data, from a Service. Is there any reason to use a bound service or intent, instead of a singleton?

I don't need to pass any data, just tell it to refresh. I'm currently doing something like:

Activity a = MyActivity.getSingleton();
if (a != null) {
    a.refreshUI();
}

Is this OK, or do I need to use a bound service, intent, etc. for some reason?