I can't find this question answered (or addressed) anywhere else, so maybe I am just missing something really obvious, but I have followed these official GCM steps to get GCM added to my Android project and have got stuck.
My first issue is that the documentation states...
...copy the Server API key. Before running the app, you'll need to add this as the value of API_KEY in line 31 of GcmSender.java.
...but what/where is GcmSender.java
?
And, my main question, how/where do I specify the URL of my application server in my Android project?
NB - I am using Android Studio. I haven't created my application server yet, but will be doing so with a PHP script that uses HTTP messaging.