How does Memory management work in android??? getting a lot of out of memory exceptions

How does Memory management work in android??? getting a lot of out of
memory exceptions

I'm learning java as I learn how to program the android. I was under the
impression tht java does all the memory management for you. But this does
not seem to be true. My small apps ran fin. I'm working on a puzzle app
for toddlers that has a lot of graphics. I'v been getting a lot of out of
memory exceptions after the app runs a while. Currently it runs fine, but
I solved the issue by setting the bitmap pointers to null when I'm done
with them. At the end of each sevic now I set all the bitmap objects to
null and its working.
It works, but I do not understand if I'm suppose to free the pointers or
java??????