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

Android will keep the process until it needs to reclaim the process memory. That way, if the user immediately returns to your application, your application will appear quicker.

The value will persist until the application is killed.

Should not using System.exit(10) followed by Process.killProcess(Process.myPid()) for exiting the application?

You should really think about not exiting the application. This is not how Android apps usually work.