Head First Java shows where HashSet fits in the API Collection tree.

Collection
(interface)
^
Set
(interface)
^
HashSet

From reading about Using a HashSet Instead of ArrayList I can tell that HashSet can hold objects.
I have to look for information on What a HashSet is and what a HashCode is.
I think that each object gets its own unique HashCode in the HashSet but that is all that I know about it.

I'll read about HashSets and read any explanation you can give me.