android memory management in activity lifecycle

Question

My question is a little bit complicated.

I want to understand how the app treats resources (especially images for backgrounds, buttons, etc.) when Activity is starte

Answer 1

Okay so we have the following situation:

A > onCreate
A > onStart
A > onResume
A > Use up a load of memory (A could even use up too much and crash)
A > Launch activity B
B > onCreate
A > onPause
B > onStart
A > onStop
B > onResume
B > Use up a load of memory 

If B uses up enough memory then the Android system will kill activity A (you'll notice the onPause and onStop methods of A have already been called so it's already been given a chance to save it's state)

If you then press the back button the Android system will just start activity A again (and if it's clever it should remember it's last state) so it looks like nothing ever happened to the user.

So to be a bit clearer: if you start B and then finish A, B will essentially just replace A in the activity stack and pressing the Back button in activity B will just exit your and app and not return to activity A.

If on the other hand you start B without finishing A, then pressing the back button in B will take you back to A. While activity A is in the background it could be killed to reclaim memory, but Android will recreate it as needed when the user navigates through the activities stack.

Also if you have in memory-caches of multiple objects (e.g. bitmaps/drawables) then back your collection by SoftReferences so that the GC can clear them up if it's running low on memory.