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

for your Update:

How can I clear the application constants on exit of the application(Back press of the HomeActivity)?

Override onBackPressed inside your HomeActivity and before call super.onBackPressed() clear all the variable. If you have a setter for a String located inside your Application subclass:

@Override
public void onBackPressed() {
    getApplication().setMyString(null);
    super.onBackPressed();
}

if you have plenty states to clear you can write a method wich reset all the fields and call it:

@Override
public void onBackPressed() {
    getApplication().clearAllMyFields();
    super.onBackPressed();
}

Of course those field have not to be marked as final or static final