InstanceIdResult Result object obtained from requests for an Instance ID and token. 


FirebaseInstanceId Firebase Instance ID provides a unique identifier for each app instance and a mechanism to authenticate and authorize actions (example: sending FCM messages). 
FirebaseInstanceIdReceiver WakefulBroadcastReceiver that receives FirebaseInstanceId and FirebaseMessaging events and delivers them to the application-specific FirebaseInstanceIdService subclass. 
FirebaseInstanceIdService This class is deprecated. In favour of overriding onNewToken in FirebaseMessagingService. Once that has been implemented, this service can be safely removed.