ReduceObservable.java

        Observable.just(1, 2, 3, 4, 5)
                .reduce(new BiFunction<Integer, Integer, Integer>() {
                    @Override
                    public Integer apply(Integer integer, Integer integer2) {
                        return (integer * integer2);
                    }
                })
                .subscribe(new MaybeObserver<Integer>() {
                    @Override
                    public void onSubscribe(Disposable d) {

                    }

                    @Override
                    public void onSuccess(Integer integer) {
                        System.out.println("Product of numbers from 1, 2, 3, 4, 5: " + integer);
                    }

                    @Override
                    public void onError(Throwable e) {

                    }

                    @Override
                    public void onComplete() {

                    }
                });