What happens when all activities of an application finishes?

Question

Scenario:

I\'ve four activities in my Android Application, lets say A, B, C and D. There is one Constants.java class in the app which extends App

Answer 1

Will finishing all activities by calling finish() of each activity will close the Application(Its process)?

No, it should not. This is because Activities are not the only component of an app. Services, BroadcastReceivers can still run without a UI. By keeping the Application process running while it can, Android ensures faster response to the user opening the app again, or faster launching of Services etc. as the Application instance doesn't need to be recreated.

Your process will only be terminated if you or another app explicitly kill it, or the system kills it to free resources.

How the value of a variable persists even if its all activities are finished(closed)?

Your variable is in the Application class, which follows a Singleton model, and hence your value will persist until the entire app process is killed.

Additionally, finish() only calls onDestroy(), which is not a deconstructor, as explained here. So even Activity variables may persist after finish has been called, as your instance is still kept around until Android feels the need to destroy it, or your process is killed.

Is it fair to call System.exit(10) followed by Process.killProcess(Process.myPid()) for exiting the application?

While this will kill the app, it is not recommended. See this question for a variety of reasons for that.