retrofit2_example_NetworkConnectionManager(onResponse).java

@Override
public void onResponse(Response<User> response, Retrofit retrofit) {
    User user = response.body();

    if (user == null) {
        //404 or the response cannot be converted to User.
        ResponseBody responseBody = response.errorBody();
        if (responseBody != null) {
            listener.onBodyError(responseBody);
        } else {
            listener.onBodyErrorIsNull();
        }
    } else {
        //200
        listener.onResponse(user, retrofit);
    }

}

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