Good example of a bad abstraction. If you're speaking the language (or "abstraction") of sets, you should see certain terminology: union, intersection, disjunction. These words are not part of the Java Set interface.
I would actually argue that the Collections API itself is a pretty good abstraction. It is often the case that conceptually I just want to work with multiple things and properties like order, duplicates, random access, etc. are not particularly important (in fact, requiring them adds inherent complexity). It's very useful that a vast amount of the standard library data structures conform to this interface or can create data views that conform to it.
