CountObservable.java

        Observable.just(1, 2, 3, 4, 5)
                .count()
                .subscribe(new SingleObserver<Long>() {
                    @Override
                    public void onSubscribe(Disposable d) {

                    }

                    @Override
                    public void onSuccess(Long aLong) {
                        System.out.println("Count: " + aLong);
                    }

                    @Override
                    public void onError(Throwable e) {

                    }
                });