Learning RxJava - Second Edition

defaultIfEmpty()

If we want to resort to a single emission when a given Observable turns out to be empty, we can use defaultIfEmpty(). For example, if we have an Observable<String> and filter only items that start with Z, we can resort to emitting None:

import io.reactivex.rxjava3.core.Observable;public class Ch3_03 {    public static void main(String[] args) {        Observable<String> items = Observable.just("Alpha", "Beta");        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 the message None. It happens only when the source Observable is empty.