threading.java

public class Stackoverflow45784477 {
    @Test
    public void name() throws Exception {
        Observable.just(1000, 500, 1300)
                .observeOn(Schedulers.single())
                .flatMap(integer -> create(integer))
                .doOnNext(s -> {
                    System.out.println(Thread.currentThread().getName());
                })
                .test()
                .awaitDone(2000, TimeUnit.MILLISECONDS)
                .assertValues("500_9", "1000_9", "1300_9");
    }

    private Observable<String> create(int sleepTime) {
        Observable<String> integerObservable = Observable.defer(() -> {
            Thread.sleep(sleepTime);

            return Observable.just(sleepTime + "_" + 9);
        }).subscribeOn(Schedulers.io());

        return integerObservable;
    }
}