Activity stack in android operating system

Activity Stack:-An android application usually contains lot of activities .

##When a user starts an activity, Android OS push that into a stack.

##If user starts another activity, then first activity goes down and newly start activity add to the top of the stack.

##When user push back button, then top activity is destroyed and the activity which is below the top activity is resumed.

For example:-
(a) When user received a message on android phone. It see a list of message in inbox, this is activity 1.

(b) When user opens a message for reading , this is activity 2.
(c) After that user reply on that message, this is activity 3.
(d) Then the press back button and get Activity 2 again. Activity 3 has been destroyed.

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