MVP

CountriesPresenter.java

public class CountriesPresenter {

  private final CountryRepository repository = new CountryRepositoryImpl();
  private CountriesActivity view;
  
  public CountriesPresenter(CountriesActivity countryActivity) {
    view = countryActivity;
  }
  
  public void getCountries() {
      repository.getCountries()
        .subscribeOn(Schedulers.io())
        .observeOn(AndroidSchedulers.mainThread())
        .subscribe(countries -> view.showCountries(countries), error -> view.showError(error));
  }
}

CountriesRepositoryImpl.java

public class CountryRepositoryImpl implements CountryRepository {...}

CountryActivtity.java

public class CountriesActivity extends Activity {
  CountriesPresenter presenter;
  
  @Override public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(...);
    presenter = new CountriesPresenter(this);
    presenter.getCountries();
  }
  
  public void showError(Thowable error) {...}
  public void shoCountries(List<Country> countries) {...}
   
}

CountryRepository.java

interface CountryRepository {
  Observable<List<Country>> getCountries();
}