CallBackExample.java

interface PlacesService {
    @GET("places/{id}")
    Call<Place> getPlaceById(@Path("id") int placeId);
}

interface UsersService {
    @GET("users")
    Call<List<User>> listUsers();
}

public class DataSource {

    // ....

    public void getUsersWithPlace(){
        usersService.listUsers()
                .enqueue(new Callback<List<User>>()
                {
                    ArrayList<String> myDataset = new ArrayList<>();
                    @Override
                    public void onResponse(@NonNull Call<List<User>> call, @NonNull Response<List<User>> response) {
                        List<User> users = response.body();
                        for (final User user : users) {
                            placesService.getPlaceById(user.placeId)
                                    .enqueue(new Callback<Place>(){
                                        @Override
                                        public void onResponse(@NonNull Call<Place> call, @NonNull Response<Place> response) {
                                            Place place = response.body();
                                            myDataset.add(user.name + " - " + place.title);
                                            updateAdapter(myDataset);
                                        }

                                        @Override
                                        public void onFailure(@NonNull Call<Place> call, @NonNull Throwable t) {
                                            Log.e("DataSource", "onFailure: ", t );
                                        }
                                    });
                        }
                    }
                    @Override
                    public void onFailure(@NonNull Call<List<User>> call, @NonNull Throwable t) {
                        Log.e("DataSource", "onFailure: ", t );
                    }
                });
    }

    // ....
}