Going back to the previous activity in android

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http://stackoverflow.com – I go from activity A to B. Now I want to go back to Activity A again. One way is that i do not finish activity A when i start activity B and then just finish Activity B. Questions: 1 If android has destroyed the activity A due to less memory or other issues, then that activity would no longer be there in the backstack and then what happens when we press the back button on Activity B. 2 How to configure an activity such that if its instance exits, then we go to it other wise we create a new instance? Thanks and regards, Sunny (HowTos)