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RxJava: combine two optional observables

I have two Observables, let's call them PeanutButter and Jelly. I'd like to combine them to a Sandwich Observable. I can do that using:
Observable<PeanutButter> peanutButterObservable = ...;
Observable<Jelly> jellyObservable = ...;
Observable<Sandwich> sandwichObservable = Observable.combineLatest(
peanutButterObservable,
jellyObservable,
(pb, j) -> makeSandwich(pb, j))
The problem is that RX waits for the first PeanutButter and the first Jelly to be emitted before emitting the first combined Sandwich but Jelly may never be emitted which means I never get the first Sandwich.
I'd like to combine the two feeds such that a combined item is emitted as soon as the first item from either feed is emitted, regardless of whether the other feed has yet to emit anything, how do I do that in RxJava?

one possible approach would be to use the startWith operator to trigger an emission of a known value from each stream upon subscription. this way combineLatest() will trigger if either stream emits a value. you'd just have to be mindful of looking out for the initial/signal values in the onNext consumer.
something like this...:
#Test
public void sandwiches() {
final Observable<String> peanutButters = Observable.just("chunky", "smooth")
.startWith("--initial--");
final Observable<String> jellies = Observable.just("strawberry", "blackberry", "raspberry")
.startWith("--initial--");
Observable.combineLatest(peanutButters, jellies, (peanutButter, jelly) -> {
return new Pair<>(peanutButter, jelly);
})
.subscribe(
next -> {
final String peanutButter = next.getFirst();
final String jelly = next.getSecond();
if(peanutButter.equals("--initial--") && jelly.equals("--initial--")) {
// initial emissions
} else if(peanutButter.equals("--initial--")) {
// jelly emission
} else if(jelly.equals("--initial--")) {
// peanut butter emission
} else {
// peanut butter + jelly emissions
}
},
error -> {
System.err.println("## onError(" + error.getMessage() + ")");
},
() -> {
System.out.println("## onComplete()");
}
);
}

I think this problem can be approached by using merge and scan operators:
public class RxJavaUnitTestJava {
public Observable<Sandwich> getSandwich(Observable<Jelly> jelly, Observable<PeanutButter> peanutButter) {
return Observable.merge(jelly, peanutButter)
.scan(new Sandwich(null, null), (BiFunction<Object, Object, Object>) (prevResult, newItem) -> {
Sandwich prevSandwich = (Sandwich) prevResult;
if (newItem instanceof Jelly) {
System.out.println("emitted: " + ((Jelly) newItem).tag);
return new Sandwich((Jelly) newItem, prevSandwich.peanutButter);
} else {
System.out.println("emitted: " + ((PeanutButter) newItem).tag);
return new Sandwich(prevSandwich.jelly, (PeanutButter) newItem);
}
})
.skip(1) // skip emitting scan's default item
.cast(Sandwich.class);
}
#Test
public void testGetSandwich() {
PublishSubject<Jelly> jelly = PublishSubject.create();
PublishSubject<PeanutButter> peanutButter = PublishSubject.create();
getSandwich(jelly, peanutButter).subscribe(new Observer<Sandwich>() {
#Override
public void onSubscribe(Disposable d) {
System.out.println("onSubscribe");
}
#Override
public void onNext(Sandwich sandwich) {
System.out.println("onNext: Sandwich: " + sandwich.toString());
}
#Override
public void onError(Throwable e) {
System.out.println("onError: " + e.toString());
}
#Override
public void onComplete() {
System.out.println("onComplete");
}
});
jelly.onNext(new Jelly("jelly1"));
jelly.onNext(new Jelly("jelly2"));
peanutButter.onNext(new PeanutButter("peanutButter1"));
jelly.onNext(new Jelly("jelly3"));
peanutButter.onNext(new PeanutButter("peanutButter2"));
}
class Jelly {
String tag;
public Jelly(String tag) {
this.tag = tag;
}
}
class PeanutButter {
String tag;
public PeanutButter(String tag) {
this.tag = tag;
}
}
class Sandwich {
Jelly jelly;
PeanutButter peanutButter;
public Sandwich(Jelly jelly, PeanutButter peanutButter) {
this.jelly = jelly;
this.peanutButter = peanutButter;
}
#Override
public String toString() {
String jellyResult = (jelly != null) ? jelly.tag : "no jelly";
String peanutButterResult = (peanutButter != null) ? peanutButter.tag : "no peanutButter";
return jellyResult + " | " + peanutButterResult;
}
}
}
Output:
onSubscribe
emitted: jelly1
onNext: Sandwich: jelly1 | no peanutButter
emitted: jelly2
onNext: Sandwich: jelly2 | no peanutButter
emitted: peanutButter1
onNext: Sandwich: jelly2 | peanutButter1
emitted: jelly3
onNext: Sandwich: jelly3 | peanutButter1
emitted: peanutButter2
onNext: Sandwich: jelly3 | peanutButter2
The fact that Jelly, PeanutButter and Sandwich are all independent types makes it a bit more complex around casting and nullability in scan. If you have control over these types, this solution can be further improved.

My Retrofit call.enque() method is getting skipped over entirely, not sure why

I'm making a call using Retrofit's enqueue() method. I'm calling my refreshImages() in my MainActivity's onCreate(), refreshImages() then calls a method refreshImagesIds() which is supposed to make a call out to Flickr's API and return back a PhotosList object, I'll then pull out the Photos from there which will contain a list of Photo objects. My issue is that for some reason the onResponse() inside my enqueue() method is never getting called. When I use the debugger it skips right over it, and when I put Log statements inside they never get written out. I know the endpoint it is hitting is correct because I can see it using OkHttp's logger, and my POJOs all look to be correct for the data being returned.
Any idea why this isn't working? Below are my refreshImages and refreshImagesId. These are both contained in my MainAcitivty and modify class-level variables.
private void refreshImages() {
// make api call
//imageUrls = FlickrServiceManager_withinterface.getKittenImages(8);
refreshImageIds();
List<Photo> photos = photosList.getPhotos().getPhoto();
imageIds = new ArrayList<String>();
for(Photo photo : photos) {
Log.d("TAG", "It is pringint imageIds: " + photo.getId());
imageIds.add(photo.getId());
}
}
private void refreshImageIds() {
Retrofit retrofit = Api.getRestAdapter();
FlickrServiceInterface flickrService = retrofit.create(FlickrServiceInterface.class);
Call<PhotosList> call = flickrService.getPhotos(API_KEY, FORMAT, "1");
imageIds = new ArrayList<String>();
call.enqueue(new Callback<PhotosList>(){
#Override
public void onResponse(Call<PhotosList> call, Response<PhotosList> response) {
photosList = response.body();
}
#Override
public void onFailure(Call<PhotosList> call, Throwable t) {
// TODO: Clean up
Log.d("TEMP_TAG", "Call failed");
}
});
}
And my FlickrServiceInterface:
public interface FlickrServiceInterface {
#GET("?method=flickr.photos.getSizes")
Call<PhotoSizes> getPhotoSizes(#Query("api_key") String apiKey, #Query("format") String format, #Query("nojsoncallback") String jsonCallback, #Query("photo_id") String photoId);
#GET("?method=flickr.photos.getRecent")
Call<PhotosList> getPhotos(#Query("api_key") String apiKey, #Query("format") String format, #Query("nojsoncallback") String jsonCallback);
}

Change your call to the synchronous retrofit API :
public static List<String> getImageIds(int size) {
Call<PhotosList> call = flickrService.getPhotos(apiKey, format, "1");
photoIds = new ArrayList<String>();
PhotosList photosList = call.execute().body();
List<Photo> photos = photosList.getPhotos().getPhoto();
for(Photo photo : photos) {
Log.d("TEMP_TAG", "adding photo id to list: " + photo.getId());
photoIds.add(photo.getId());
}
Log.d("TEMP_TAG", "it's getting here too");
return photoIds;
}
Please note that you need to call this method on an AsyncTask
EDIT
You could also continue to use enqueue, but you need to provide an "onFinish" hook, so you know when your data has been received and then you "notify" the client with the data:
//interface por communication
public interface ImageIdsCallBack {
public void onFinish( List<String> photoIds );
}
Then you receive this interface and send data:
public static List<String> getImageIds(int size, final ImageIdsCallBack callback) {
Call<PhotosList> call = flickrService.getPhotos(apiKey, format, "1");
photoIds = new ArrayList<String>();
call.enqueue(new Callback<PhotosList>(){
#Override
public void onResponse(Call<PhotosList> call, Response<PhotosList> response) {
PhotosList photosList = response.body();
List<Photo> photos = photosList.getPhotos().getPhoto();
for(Photo photo : photos) {
Log.d("TEMP_TAG", "adding photo id to list: " + photo.getId());
photoIds.add(photo.getId());
}
//send the data to the caller
callback.onFinish(photoIds);
}
#Override
public void onFailure(Call<PhotosList> call, Throwable t) {
// TODO: Clean up
Log.d("TEMP_TAG", "Call failed");
}
});
Log.d("TEMP_TAG", "it's getting here too");
return photoIds;
}
calling the method :
getImageIds( 50 , new ImageIdsCallBack() {
public void onFinish( List<String> photoIds ) {
//update UI with photoIds
}
} );
I typically use a library like EventBus to make it easier, I really recommend it to you.

Correct me if I'm wrong, is this on the main thread? That would pose the problem of not waiting for a response.
Consider using async

rxjava - getting response and inserting diff parallelly

I am using rxjava for parallel processing of two requests using Observable.zip. What I am trying to do is , in one observable say response I am getting one response and in other observable say diff I am trying to get the response and save this difference in DB. The problem is I am not sure how to achieve my requirement as the diff observable is not getting completed if response observable gets the response
Here is what I am doing ...
public ServiceResponse getDummyResponse(ServiceRequest serviceRequest, String prodId){
Observable<ServiceResponse> subInfoDummyObservable = getDummyResonseGenericObservable();
Observable<ServicesDiff> reObservable = getServicesDiffGenericObservable(serviceRequest, prodId);
Observable<ServiceResponse> responseObservable = Observable.zip(
subInfoDummyObservable,
reObservable,
new Func2<ServiceResponse, ServicesDiff, ServiceResponse>() {
#Override
public ServiceResponse call(ServiceResponse serviceResponse, ServicesDiff diffResponse) {
return serviceResponse;
}
}
);
ServiceResponse serviceResponse = responseObservable.toBlocking().single();
return serviceResponse;
}
Observable<ServiceResponse> getDummyResonseGenericObservable() {
return GenericHystrixCommand.toObservable("getDummyResonseGenericObservable", "getDummyResonseGenericObservable", () -> new ServiceResponse(),(t) -> {return null;} );
}
Observable<ServicesDiff> getServicesDiffGenericObservable(ServiceRequest serviceRequest, String prodId) {
return GenericHystrixCommand.toObservable("getServicesDiffGenericObservable", "getServicesDiffGenericObservable", () -> getBothServiceResponses(serviceRequest, prodId),(t) -> {return null;} );
}
public ServicesDiff getBothServiceResponses(ServiceRequest serviceRequest, String prodId) {
Observable<String> service1ResponseObservable = getService1GenericObservable(prodId);
Observable<ServiceResponse> service2ResponseObservable = getService2GenericObservable(serviceRequest, prodId);
Observable<ServicesDiff> observable = Observable.zip(
service1ResponseObservable, service2ResponseObservable,
new Func2<String, ServiceResponse, ServicesDiff>() {
#Override
public ServicesDiff call(String service1Response, ServiceResponse service2Response) {
return aggregate(service1Response, service2Response); // never reaches this point**********
}
}
);
ServicesDiff response = observable.toBlocking().single();
return response;
}
I am inserting the diff to DB in aggregate method but it never reaches to aggregate at all. Please let me know what I am doing wrong here? Thanks.

Observable are a description of how to consume data. In your code sample, you don't subscribe, you don't actually consume the data. You just described how to request, but the subscribe part, the part that trigger the requests, is missing.
So if I rewrite a little your code:
class Aggregate {
Aggregate(String reponse, ServicesDiff diff) {
...
}
}
Observable<String> getService1GenericObservable(String prodId) {
...
}
Observable<ServicesDiff> getServicesDiffGenericObservable(ServiceRequest serviceRequest, String prodId) {
...
}
public Observable<Aggregate> getBothServiceResponses(ServiceRequest serviceRequest, String prodId) {
Observable<String> service1ResponseObservable = getService1GenericObservable(prodId);
Observable<ServiceResponse> service2ResponseObservable = getService2GenericObservable(serviceRequest, prodId);
return Observable<Aggregate> observable = Observable.zip(
service1ResponseObservable, service2ResponseObservable,
new Func2<String, ServiceResponse, ServicesDiff>() {
#Override
public ServicesDiff call(String service1Response, ServiceResponse service2Response) {
return aggregate(service1Response, service2Response);
}
}
);
}
You will just have to do this to access the result aggregate:
getBothServiceResponses(serviceRequest, prodId).subscribe(...)

How to rerun CompositeSubscription with ReplaySubject?

I have CompositeSubscription , and there I add Subscription with ReplaySubject
CompositeSubscription compositeSubscription = new CompositeSubscription();
ReplaySubject subject = ReplaySubject.create();
compositeSubscription.add(
manager.getAllContacts()
.toList()
.doOnNext(new Action1<List<Person>>() {
#Override
public void call(List<Person> persons) {
allPersons = persons;
Log.e(TAG, "BookContacts: " + "allPersons = " + allPersons.size());
setupViewPager();
}
})
.subscribe(subject));
then I add second Subscription with this ReplaySubject
compositeSubscription.add(Observable.combineLatest(subject,
(PublishSubject<List<CustomUser>>) execute(
manager.getDigitsContacts()),
new Func2<List<Person>, List<CustomUser>, Object>() {
#Override
public Object call(List<Person> persons, List<CustomUser> customUsers) {
//... my code with persons and customUsers...
return null;
}
})
.subscribe());
code is working, after that complete ReplaySubject hasCompleted = true.
but when I try to add third Subscription , it doesn't call "call()" method
compositeSubscription.add(Observable.combineLatest(subject,
(PublishSubject<List<CustomUser>>) execute(
manager.getFacebookContacts()), //<-----manager.getFacebookContacts() is run, but doesn't call call() method
new Func2<List<Person>, List<CustomUser>, Object>() {
#Override
public Object call(List<Person> persons, List<CustomUser> customUsers) {
//...this method is not called after "manager.getFacebookContacts()"
return null;
}
})
.subscribeOn(Schedulers.newThread())
.subscribe());
HOW TO SOLVE IT?...Because if I add Subscription simultaneously it works fine.

Could you please add error callback to .subscribe()? My guess is that the third time, ReplaySubject overflows the combineLatest's buffer. Instead of creating a subject, you should use .replay().autoConnect(0)
CompositeSubscription compositeSubscription = new CompositeSubscription();
Observable<List<Person>> persons = manager.getAllContacts()
.toList()
.doOnNext(new Action1<List<Person>>() {
#Override
public void call(List<Person> persons) {
allPersons = persons;
Log.e(TAG, "BookContacts: " + "allPersons = " + allPersons.size());
setupViewPager();
}
}).replay().autoConnect(0, s -> compositeSubscription.add(s));
Then use persons instead of subject

Rxjava Android how to use the Zip operator

I am having a lot of trouble understanding the zip operator in RxJava for my android project.
Problem
I need to be able to send a network request to upload a video
Then i need to send a network request to upload a picture to go with it
finally i need to add a description and use the responses from the previous two requests to upload the location urls of the video and picture along with the description to my server.
I assumed that the zip operator would be perfect for this task as I understood we could take the response of two observables (video and picture requests) and use them for my final task.
But I cant seem to get this to occur how I envision it.
I am looking for someone to answer how this can be done conceptually with a bit of psuedo code.
Thank you

Zip operator strictly pairs emitted items from observables. It waits for both (or more) items to arrive then merges them. So yes this would be suitable for your needs.
I would use Func2 to chain the result from the first two observables.
Notice this approach would be simpler if you use Retrofit since its api interface may return an observable. Otherwise you would need to create your own observable.
// assuming each observable returns response in the form of String
Observable<String> movOb = Observable.create(...);
// if you use Retrofit
Observable<String> picOb = RetrofitApiManager.getService().uploadPic(...),
Observable.zip(movOb, picOb, new Func2<String, String, MyResult>() {
#Override
public MyResult call(String movieUploadResponse, String picUploadResponse) {
// analyze both responses, upload them to another server
// and return this method with a MyResult type
return myResult;
}
}
)
// continue chaining this observable with subscriber
// or use it for something else

A small example:
val observableOne = Observable.just("Hello", "World")
val observableTwo = Observable.just("Bye", "Friends")
val zipper = BiFunction<String, String, String> { first, second -> "$first - $second" }
Observable.zip(observableOne, observableTwo, zipper)
.subscribe { println(it) }
This will print:
Hello - Bye
World - Friends
In BiFunction<String, String, String> the first String the type of the first observable, the second String is the type of the second observable, the third String represents the type of the return of your zipper function.
I made a small example that calls two real endpoints using zip in this blog post

Here I have an example that I did using Zip in asynchronous way, just in case you´re curious
/**
* Since every observable into the zip is created to subscribeOn a diferent thread, it´s means all of them will run in parallel.
* By default Rx is not async, only if you explicitly use subscribeOn.
*/
#Test
public void testAsyncZip() {
scheduler = Schedulers.newThread();
scheduler1 = Schedulers.newThread();
scheduler2 = Schedulers.newThread();
long start = System.currentTimeMillis();
Observable.zip(obAsyncString(), obAsyncString1(), obAsyncString2(), (s, s2, s3) -> s.concat(s2)
.concat(s3))
.subscribe(result -> showResult("Async in:", start, result));
}
/**
* In this example the the three observables will be emitted sequentially and the three items will be passed to the pipeline
*/
#Test
public void testZip() {
long start = System.currentTimeMillis();
Observable.zip(obString(), obString1(), obString2(), (s, s2, s3) -> s.concat(s2)
.concat(s3))
.subscribe(result -> showResult("Sync in:", start, result));
}
public void showResult(String transactionType, long start, String result) {
System.out.println(result + " " +
transactionType + String.valueOf(System.currentTimeMillis() - start));
}
public Observable<String> obString() {
return Observable.just("")
.doOnNext(val -> {
System.out.println("Thread " + Thread.currentThread()
.getName());
})
.map(val -> "Hello");
}
public Observable<String> obString1() {
return Observable.just("")
.doOnNext(val -> {
System.out.println("Thread " + Thread.currentThread()
.getName());
})
.map(val -> " World");
}
public Observable<String> obString2() {
return Observable.just("")
.doOnNext(val -> {
System.out.println("Thread " + Thread.currentThread()
.getName());
})
.map(val -> "!");
}
public Observable<String> obAsyncString() {
return Observable.just("")
.observeOn(scheduler)
.doOnNext(val -> {
System.out.println("Thread " + Thread.currentThread()
.getName());
})
.map(val -> "Hello");
}
public Observable<String> obAsyncString1() {
return Observable.just("")
.observeOn(scheduler1)
.doOnNext(val -> {
System.out.println("Thread " + Thread.currentThread()
.getName());
})
.map(val -> " World");
}
public Observable<String> obAsyncString2() {
return Observable.just("")
.observeOn(scheduler2)
.doOnNext(val -> {
System.out.println("Thread " + Thread.currentThread()
.getName());
})
.map(val -> "!");
}
You can see more examples here https://github.com/politrons/reactive

zip operator allow you to compose a result from results of two different observable.
You 'll have to give am lambda that will create a result from datas emitted by each observable.
Observable<MovieResponse> movies = ...
Observable<PictureResponse> picture = ...
Observable<Response> response = movies.zipWith(picture, (movie, pic) -> {
return new Response("description", movie.getName(), pic.getUrl());
});

i have been searching for a simple answer on how to use the Zip operator, and what to do with the Observables i create to pass them to it, i was wondering if i should call subscribe() for every observable or not, non of these answers were simple to find, i had to figure it out by my self, so here is a simple example for using Zip operator on 2 Observables :
#Test
public void zipOperator() throws Exception {
List<Integer> indexes = Arrays.asList(0, 1, 2, 3, 4);
List<String> letters = Arrays.asList("a", "b", "c", "d", "e");
Observable<Integer> indexesObservable = Observable.fromIterable(indexes);
Observable<String> lettersObservable = Observable.fromIterable(letters);
Observable.zip(indexesObservable, lettersObservable, mergeEmittedItems())
.subscribe(printMergedItems());
}
#NonNull
private BiFunction<Integer, String, String> mergeEmittedItems() {
return new BiFunction<Integer, String, String>() {
#Override
public String apply(Integer index, String letter) throws Exception {
return "[" + index + "] " + letter;
}
};
}
#NonNull
private Consumer<String> printMergedItems() {
return new Consumer<String>() {
#Override
public void accept(String s) throws Exception {
System.out.println(s);
}
};
}
the printed result is :
[0] a
[1] b
[2] c
[3] d
[4] e
the final answers to the questions that where in my head were as follows
the Observables passed to the zip() method just need to be created only, they do not need to have any subscribers to them, only creating them is enough ... if you want any observable to run on a scheduler, you can specify this for that Observable ... i also tried the zip() operator on Observables where they should wait for there result, and the Consumable of the zip() was triggered only when both results where ready (which is the expected behavior)

This is my implementation using Single.zip and rxJava2
I tried to make it as easy to understand as possible
//
// API Client Interface
//
#GET(ServicesConstants.API_PREFIX + "questions/{id}/")
Single<Response<ResponseGeneric<List<ResponseQuestion>>>> getBaseQuestions(#Path("id") int personId);
#GET(ServicesConstants.API_PREFIX + "physician/{id}/")
Single<Response<ResponseGeneric<List<ResponsePhysician>>>> getPhysicianInfo(#Path("id") int personId);
//
// API middle layer - NOTE: I had feedback that the Single.create is not needed (but I haven't yet spent the time to improve it)
//
public Single<List<ResponsePhysician>> getPhysicianInfo(int personId) {
return Single.create(subscriber -> {
apiClient.getPhysicianInfo(appId)
.subscribeOn(Schedulers.io())
.observeOn(Schedulers.io())
.subscribe(response -> {
ResponseGeneric<List<ResponsePhysician>> responseBody = response.body();
if(responseBody != null && responseBody.statusCode == 1) {
if (!subscriber.isDisposed()) subscriber.onSuccess(responseBody.data);
} else if(response.body() != null && response.body().status != null ){
if (!subscriber.isDisposed()) subscriber.onError(new Throwable(response.body().status));
} else {
if (!subscriber.isDisposed()) subscriber.onError(new Throwable(response.message()));
}
}, throwable -> {
throwable.printStackTrace();
if(!subscriber.isDisposed()) subscriber.onError(throwable);
});
});
}
public Single<List<ResponseQuestion>> getHealthQuestions(int personId){
return Single.create(subscriber -> {
apiClient.getBaseQuestions(personId)
.subscribeOn(Schedulers.io())
.observeOn(Schedulers.io())
.subscribe(response -> {
ResponseGeneric<List<ResponseQuestion>> responseBody = response.body();
if(responseBody != null && responseBody.data != null) {
if (!subscriber.isDisposed()) subscriber.onSuccess(response.body().data);
} else if(response.body() != null && response.body().status != null ){
if (!subscriber.isDisposed()) subscriber.onError(new Throwable(response.body().status));
} else {
if (!subscriber.isDisposed()) subscriber.onError(new Throwable(response.message()));
}
}, throwable -> {
throwable.printStackTrace();
if(!subscriber.isDisposed()) subscriber.onError(throwable);
});
});
}
//please note that ResponseGeneric is just an outer wrapper of the returned data - common to all API's in this project
public class ResponseGeneric<T> {
#SerializedName("Status")
public String status;
#SerializedName("StatusCode")
public float statusCode;
#SerializedName("Data")
public T data;
}
//
// API end-use layer - this gets close to the UI so notice the oberver is set for main thread
//
private static class MergedResponse{// this is just a POJO to store all the responses in one object
public List<ResponseQuestion> listQuestions;
public List<ResponsePhysician> listPhysicians;
public MergedResponse(List<ResponseQuestion> listQuestions, List<ResponsePhysician> listPhysicians){
this.listQuestions = listQuestions;
this.listPhysicians = listPhysicians;
}
}
// example of Single.zip() - calls getHealthQuestions() and getPhysicianInfo() from API Middle Layer
private void downloadHealthQuestions(int personId) {
addRxSubscription(Single
.zip(getHealthQuestions(personId), getPhysicianInfo(personId), MergedResponse::new)
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.subscribe(response -> {
if(response != null) {
Timber.i(" - total health questions downloaded %d", response.listQuestions.size());
Timber.i(" - physicians downloaded %d", response.listPhysicians.size());
if (response.listPhysicians != null && response.listPhysicians.size()>0) {
// do your stuff to process response data
}
if (response.listQuestions != null && response.listQuestions.size()>0) {
// do your stuff to process response data
}
} else {
// process error - show message
}
}, error -> {
// process error - show network error message
}));
}

You use the zip from rxjava with Java 8:
Observable<MovieResponse> movies = ...
Observable<PictureResponse> picture = ...
Observable<ZipResponse> response = Observable.zip(movies, picture, ZipResponse::new);
class ZipResponse {
private MovieResponse movieResponse;
private PictureResponse pictureResponse;
ZipResponse(MovieResponse movieResponse, PictureResponse pictureResponse) {
this.movieResponse = movieResponse;
this.pictureResponse = pictureResponse;
}
public MovieResponse getMovieResponse() {
return movieResponse;
}
public void setMovieResponse(MovieResponse movieResponse) {
this.movieResponse= movieResponse;
}
public PictureResponse getPictureResponse() {
return pictureResponse;
}
public void setPictureResponse(PictureResponse pictureResponse) {
this.pictureResponse= pictureResponse;
}
}

You can use .zipWith operator for Observable chains.
If uploadMovies() and uploadPictures() return Observable,
uploadMovies()
.zipWith(uploadPictures()) { m, p ->
"$m with $p were uploaded"
}
.subscribe { print(it) }