The warning is saying that you are using the method onFetchData of the listener type: OnFetchDataListener without declaring the type of data it's listening for. I think OnFetchDataListener is a templated type. The warning is stating that the compiler is unable to check that onFetchData will return the type of data you want because you are declaring listener
to be a raw (untemplated) OnFetchDataListener.
My guess would be that OnFetchDataListener wants to retrieve a list of things like: ArrayList<string> or List<newsarticle> (if NewsArticle were a defined type or class object).
I think the declaration of the method getNewsHeadlines should be changed to something like the following:
public void getNewsHeadlines(OnFetchDataListener<List<String>> listener, String category, String query)
I'm not sure exactly what the template parameters for OnFetchDataListener should be. You'll have to go back into your code to determine that yourself.