switchIfEmpty()

Similar to defaultIfEmpty(), switchIfEmpty() specifies a different Observable to emit values from if the source Observable is empty. This allows you specify a different sequence of emissions in the event that the source is empty rather than emitting just one value.

We could choose to emit three additional strings, for example, if the preceding Observable came out empty due to a filter() operation:

import io.reactivex.Observable;

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

        Observable.just("Alpha", "Beta", "Gamma", "Delta", "Epsilon")
          .filter(s -> s.startsWith("Z"))
          .switchIfEmpty(Observable.just("Zeta", "Eta", "Theta"))
          .subscribe(i -> System.out.println("RECEIVED: " + i),
          e -> System.out.println("RECEIVED ERROR: " + e)
          );
     }
}

The output of the preceding code snippet is as follows:

RECEIVED: Zeta
RECEIVED: Eta
RECEIVED: Theta

Of course, if the preceding Observable is not empty, then switchIfEmpty() will have no effect and not use that specified Observable.