Why do many Collection classes in Java extend the abstract class and implement the interface as well?

Question

Why do many Collection classes in Java extend the Abstract class and also implement the interface (which is also implemented by the given abstract class)?

For example, class HashSet extends AbstractSet and also implements Set, but AbstractSet already implements Set.

Answer 1

It's a way to remember that this class really implements that interface.
It won't have any bad effect and it can help to understand the code without going through the complete hierarchy of the given class.