retrofit2_example_NetworkConnectionManager.java

public class NetworkConnectionManager {

    public NetworkConnectionManager() {

    }

    public void callServer(final OnNetworkCallbackListener listener,String username){

        Retrofit retrofit = new Retrofit.Builder()
                .baseUrl("https://api.github.com")
                .addConverterFactory(GsonConverterFactory.create())
                .build();

        GitHubService git = retrofit.create(GitHubService.class);
        Call call = git.getUser(username);
        call.enqueue(new Callback<User>() {

            @Override
            public void onResponse(Response<User> response, Retrofit retrofit) {

            }

            @Override
            public void onFailure(Throwable t) {
            }
        });

    }
}