How to add RxJava return type support in Retrofit?

Add the dependency converter-rxjava2 (or converter-rxjava3 for RxJava 3) in build.gradle:

implementation 'com.squareup.retrofit2:adapter-rxjava2:2.9.0' // or adapter-rxjava3 for RxJava 3

When creating a Retrofit instance, add RxJava2CallAdapterFactory (or RxJava3CallAdapterFactory):

Retrofit retrofit = new Retrofit.Builder()
    .baseUrl("https://api.example.com/")
    .addConverterFactory(GsonConverterFactory.create()) // Or another converter
    .addCallAdapterFactory(RxJava2CallAdapterFactory.create()) // Or RxJava3CallAdapterFactory
    .build();

In the Api Service interface, define methods that return RxJava types (Observable, Single, Completable):

public interface ApiService {
    @GET("users")
    Observable<List<User>> getUsers();

    @GET("users/{id}")
    Single<User> getUserById(@Path("id") int userId);

    @POST("users")
    Completable createUser(@Body User user);
}