public interface GitHubService {
  @GET("users/{user}/repos")
  Call<List<Repo>> listRepos(@Path("user") String user);
}

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

    GitHubService service = retrofit.create(GitHubService .class);

    Call<List<Repo>> call = service.listRepos("user");
    call.enqueue(new Callback<List<Repo>>() {
        @Override
        public void onResponse(Call<List<Repo>> call, Response<List<Repo>> response) {
            for (Repo repo:
                    response.body()) {
                    adapter.add(repo);
            }
        }
        @Override
        public void onFailure(Call<List<Repo>> call, Throwable t) {
        }
    });