Through Android's Activities doc, it is said that the methods onStop() and onDestroy() are not guaranteed to be called.
[...] once the activity is created, onPause() is the last method that's guaranteed to be called before the process can be killed—if the system must recover memory in an emergency, then onStop() and onDestroy() might not be called [...]
I would like to know, when this situation occurs, is the app also killed within the activities or just the activity itself is killed?
Answer is app process is also get killed and can be recreated. https://developer.android.com/training/basics/activity-lifecycle/recreating.html Please check http://www.vogella.com/tutorials/AndroidLifeCycle/article.html Application with only stopped activities and without a service or executing receiver. Android keeps them in a least recent used (LRU) list and if requires terminates the one which was least used.