import reactor.core.publisher.Mono;

public class test_mono {
    public static void main(String[] args) {
        Mono<String> source = Mono.empty();
        Mono<String> m = source
            .flatMap(s -> Mono.just("from flatMap " + s))
            .switchIfEmpty(Mono.defer(() -> Mono.just("from switchIfEmpty")));
            
        m.subscribe(System.out::println);
    }
}