AsyncOrderSafeObservableTest.java

public void concatEager() {

    final TestSubscriber<List<String>> testSubscriber = new TestSubscriber<>();

    final long startTime = new Date().getTime();
    System.out.println("Start.");

    Observable.concatEager( //
        asyncObservable("uno", 4), //
        asyncObservable("dos", 3), //
        asyncObservable("tres", 2), //
        asyncObservable("cuatro", 1))
        .toList()
        .doOnNext(strings -> System.out.println("onNext: " + strings))
        .doOnCompleted(() -> System.out.println("onCompleted"))
        .doOnError(Throwable::printStackTrace)
        .subscribe(testSubscriber);

    testSubscriber.awaitTerminalEvent();
    testSubscriber.assertCompleted();
    testSubscriber.assertNoErrors();

    final long endTime = new Date().getTime();
    System.out.println("End: " + (endTime - startTime) + "ms");
  }

  private Observable<String> asyncObservable(String msg, int wait) {
    return Observable.just(msg)
        .delay(wait, TimeUnit.SECONDS, Schedulers.newThread())
        .doOnNext(s -> System.out.println("onNext(src) " + s));
  }

output

12-16 20:08:26.142 I/System.out: Start.
12-16 20:08:27.148 I/System.out: onNext(src) cuatro
12-16 20:08:28.148 I/System.out: onNext(src) tres
12-16 20:08:29.147 I/System.out: onNext(src) dos
12-16 20:08:30.147 I/System.out: onNext(src) uno
12-16 20:08:30.149 I/System.out: onNext: [uno, dos, tres, cuatro]
12-16 20:08:30.149 I/System.out: onCompleted
12-16 20:08:30.158 I/System.out: End: 4015ms