Observable.empty

empty.java

Observable<String> o1 = Observable.just("Not Empty");
    Observable<String> o2 = Observable.empty();
    Observable.zip(o1, o2, (o, o21) -> {
      Timber.e(">>>>> Will not trigger" + o + ", " + o21);
      return "";
    }).subscribe();

    Observable.empty().subscribe(o -> {
      Timber.e(">>>> Will not trigger");
    });

https://stackoverflow.com/questions/43065527/observable-empty-what-returns-inside-a-zip