List<User> userList = dataAPI.getAllUser();

public class DataAPI {
public DataAPI(){
        retrofit = new Retrofit.Builder()
                .baseUrl(BASE_URL)
                .addConverterFactory(GsonConverterFactory.create())
                .addCallAdapterFactory(RxJavaCallAdapterFactory.create())
                .build();

        userAPI = retrofit.create(UserService.class);
public class DataAPI {
    [...]
    private UsertService userAPI;

    public DataAPI(){
        retrofit = new Retrofit.Builder()
                .baseUrl(BASE_URL)
                .addConverterFactory(GsonConverterFactory.create())
                .addCallAdapterFactory(RxJavaCallAdapterFactory.create())
                .build();

        userAPI = retrofit.create(UserService.class);
   }

   Observable<List<User>> getAllUser() {
        return userAPI.getAllUser();
    }
}

@GET("/user/all")
    Observable<List<User>> getAllUser();