Retrofit2 comes with support for multiple pluggable execution mechanisms, one of them is RxJava.

To use retrofit with RxJava you first need to add the Retrofit RxJava adapter to your project:

compile 'com.squareup.retrofit2:adapter-rxjava:2.1.0'

then you need to add the adapter when building your retrofit instance:

Retrofit retrofit = new Retrofit.Builder()
    .baseUrl("https://api.example.com")
    .addCallAdapterFactory(RxJavaCallAdapterFactory.create())
    .build();

In your interface when you define the API the return type should be Observable eg:

public interface GitHubService {
  @GET("users/{user}/repos")
  Observable<List<Repo>> listRepos(@Path("user") String user);
}

You can also use Single instead of Observable.

RxJava is handy when making serial request. If you want to use the result from one request to make another you can use the flatMap operator:

api.getRepo(repoId).flatMap(repo -> api.getUser(repo.getOwnerId())
    .subscribe(/*do something with the result*/);

You can use the zip operator to make request in parallel and combine the results eg:

Observable.zip(api.getRepo(repoId1), api.getRepo(repoId2), (repo1, repo2) ->
    {
        //here you can combine the results
    }).subscribe(/*do something with the result*/);