ConcatObservable.java

        Observable<String> alphabets1 = Observable
                .interval(1, TimeUnit.SECONDS).map(id -> "A" + id);

        Observable<String> alphabets2 = Observable
                .interval(1, TimeUnit.SECONDS).map(id -> "B" + id);

        Observable.concat(alphabets1, alphabets2)
                .subscribe(new Observer<String>() {
                    @Override
                    public void onSubscribe(Disposable d) {

                    }

                    @Override
                    public void onNext(String s) {
                        System.out.println("onNext: " + s);
                    }

                    @Override
                    public void onError(Throwable e) {

                    }

                    @Override
                    public void onComplete() {

                    }
                });