FlowableObservable.java

        Flowable.range(10, 1000)
                .subscribeOn(Schedulers.io())
                .reduce(1, new BiFunction<Integer, Integer, Integer>() {
                    @Override
                    public Integer apply(Integer integer, Integer integer2) {
                        return (integer + integer2);
                    }
                })
                .subscribe(new SingleObserver<Integer>() {
                    @Override
                    public void onSubscribe(Disposable d) {
                        System.out.println("onSubscribe is called");
                    }

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

                    @Override
                    public void onError(Throwable e) {
                        System.out.println("Error: " + e.getMessage());
                    }
                });