Page 102 of your book

Observable.java

// Question is totally stupid but...

// Example on the book:
Observable<Observable<String>> delay = just(
                alice.map(w -> "MOO Alice: " + w),
                paolo.map(w -> "MOO Paolo: " + w),
                carmelo.map(w -> "MOO Carmelo: " + w)
)
                .flatMap(Observable::just)
                .delay(random.nextInt(5), SECONDS);

// what is the difference with:
Observable<Observable<String>> delay = just(
                alice.map(w -> "MOO Alice: " + w),
                paolo.map(w -> "MOO Paolo: " + w),
                carmelo.map(w -> "MOO Carmelo: " + w)
)
                .delay(random.nextInt(5), SECONDS);

// Running with
switchOnNext(delay).subscribe(out::println);
// gives the same results...
// why using the flatMap?