Observable.just(AssetsUtil.getCountryList(context))
                .flatMapIterable(new Function<List<Country>, Iterable<?>>() {
                    @Override
                    public Iterable<?> apply(List<Country> countryList) throws Exception {
                        return countryList;
                    }
                })
                .subscribeOn(Schedulers.io())
                .observeOn(AndroidSchedulers.mainThread())
                .subscribe(item -> mView.onCountryListReceive((Country) item));

@Override
public void onCountryListReceive(Country country) {
    countries.add(country);
    PreferencesUtil.putCountryList(getActivity(), countries);
}

public static List<Country> getCountryList(Context context) {
    String countryJson = loadJSONFromAsset(context);

    List<Country> countryList = new Gson().fromJson(countryJson, new TypeToken<List<Country>>(){}.getType());

    cityList = new ArrayList<>();

    if (countryList != null) {
        for (Country country : countryList){
            cityList.addAll(country.getCities());
        }
    }

    return countryList;
}