defaultIfEmpty()

If we want to resort to a single emission if a given Observable comes out empty, we can use defaultIfEmpty(). For a given Observable<T>, we can specify a default T emission if no emissions occur when onComplete() is called.

If we have an Observable<String> and filter for items that start with Z but no items meet this criteria, we can resort to emitting None:

import io.reactivex.Observable;

public class Launcher {
      public static void main(String[] args) {

        Observable<String> items =
          Observable.just("Alpha","Beta","Gamma","Delta","Epsilon");

        items.filter(s -> s.startsWith("Z"))
          .defaultIfEmpty("None")
          .subscribe(System.out::println);
     }
}

The output of the preceding code snippet is as follows:

None

Of course, if emissions were to occur, we would never see None emitted. It will only happen if the preceding Observable is empty.