This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
Another thing, You should use your best judgement on when to collect garbage manually or to let Java do it all for you. If you are doing a regular program that does not consume many computer resources, then you don't need to bother your head about it, but if you are making a game or any program that is likely to take tons of comp resources, then I strongly suggest you clear out no longer needed memory allocations as soon as they are no longer needed. (This will improve performance, which is very important for games)