I wouldn't call an interface file name as ...Interface. It is already interface and pretty obvious.
So for the api folder. I would recommend you some renaming refactor operations.
As per retrofit namings and their documentation in here public interface GitHubService { @GET("users/{user}/repos") Call<List<Repo>> listRepos(@Path("user") String user); }
Rename your ApiInterface.java to CountryService.java
The class APIClient could be RetrofitBuilder to build retrofit instance for a given base_url. Or even no need for it. Retrofit is already providing a one liner builder pattern to get you an instance.