concatMap.java

@Test
public void switchMap() throws Exception {
    final List<String> items = Lists.newArrayList("a", "b", "c", "d", "e", "f");

    final TestScheduler scheduler = new TestScheduler();

    Observable.from(items)
            .concatMap( s -> {
                final int delay = new Random().nextInt(10);
                return Observable.just(s + "x")
                        .delay(delay, TimeUnit.SECONDS, scheduler);
            })
            .toList()
            .doOnNext(System.out::println)
            .subscribe();

    scheduler.advanceTimeBy(1, TimeUnit.MINUTES);
}