I am just starting to learn how to use Hibernate and there are a few things about Collections which I don't understand.

If I have an attribute List inside my class why do i have to initialize it with ArrayList? Can I use a LinkedList or another type of list?
Why do I have to initialize a Set with HashSet? Why a map with HashMap? etc.

Thanks!

Set, List are interface that you cannot instansiate, you need the concrete implementation such as HashSet, ArrayList etc.

Yes, I understand but if I have a List interface attribute in my class can I initialize it with LinkedList (concrete implementation) instead of an ArrayList? I made a very simple example using a LinkedList and I think it works but the book "Java Persistence with Hibernate" says that if I use List interface I have to initialize it with an ArrayList.