SumObservable.java

        Observable<Integer> observable = Observable
                .just(1, 2, 3, 4, 5);

        MathObservable
                .sumInt(observable)
                .subscribe(new Observer<Integer>() {
                    @Override
                    public void onSubscribe(Disposable d) {

                    }

                    @Override
                    public void onNext(Integer integer) {
                        System.out.println("Sum: " + integer);
                    }

                    @Override
                    public void onError(Throwable e) {

                    }

                    @Override
                    public void onComplete() {

                    }
                });