Observable just转换from

Observable.just(10)
                .flatMap(new Func1<Integer, Observable<String>>() {
                    @Override
                    public Observable<String> call(Integer integer) {
                        ArrayList<String> arr = new ArrayList<>();
                        for (int i = 0; i < integer.intValue(); i++) {
                            arr.add(String.valueOf(i));
                        }
                        return Observable.from(arr);
                    }
                })
                .doOnNext(new Action1<String>() {
                    @Override
                    public void call(String s) {
                        Log.e("Observable", s);
                    }
                })
                .subscribe();

  Observable.create(new Observable.OnSubscribe<List<String>>() {

                    @Override
                    public void call(Subscriber<? super List<String>> subscriber) {
                        ArrayList<String> arr = new ArrayList<>();
                        for (int i = 0; i < 10; i++) {
                            arr.add(String.valueOf(i));
                        }
                        subscriber.onNext(arr);
                    }
                })
                .flatMap(new Func1<List<String>, Observable<String>>() {
                    @Override
                    public Observable<String> call(List<String> strings) {
                        return Observable.from(strings);
                    }
                })
                .doOnNext(new Action1<String>() {
                    @Override
                    public void call(String s) {
                        Log.e("Observable", s);
                    }
                })
                .subscribe();