ServiceCaller.java

public class ServiceCaller {

    private static final String BASE_URL = "https://example.com/";

    public static void startLicense(RequestLicense requestLicense, Callback<ResponseFromService> callback) {

        Gson gson = new GsonBuilder()
                .setLenient()
                .create();

        Retrofit retrofit = new Retrofit.Builder()
                .baseUrl(BASE_URL)
                .addConverterFactory(GsonConverterFactory.create(gson))
                .build();

        WebServiceAPI wsAPI = retrofit.create(WebServiceAPI.class);

        Call<ResponseFromService> call = wsAPI.sendDataForLicense(requestLicense);
        call.enqueue(callback);
    }