Retrofit Client and Interface

public interface ApiInterface {

    @POST("/iuk/api/credentials")
    Call<ServerResponse> operation(@Body ServerRequest request);
}


public class RetrofitApiClient {
    private static final String BASE_URL = "https://xyz.000webhostapp.com";
    private static Retrofit retrofit = null;

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

    private RetrofitApiClient() {}

    public static synchronized Retrofit getClient() {
        if (retrofit==null) {
            retrofit = new Retrofit.Builder()
                    .baseUrl(Config_Ref.BASE_URL)
                    .addConverterFactory(GsonConverterFactory.create(gson))
                    .build();
        }
        return retrofit;
    }
}