Can you copyright Harry Potter the character? Or can anyone use recognizable Harry Potter reproductions in commercial context, be it alternative books, movies or merchandise?
This is where the confusion lies: Google claims Oracle is copyrighting an interface to Harry Potter (as per your example) -- i.e., they are claiming copyright to any / all characters that are (1) a boy in his early teens (2) has magical powers (3) goes to wizard school.
Now, the question is, where do we draw the line: As per Oracle, there cannot be any other character that does what Harry Potter does.
Copyright law already allows you to copyright specific combinations of unprotectable elements. It's called thin copyright, it's why Katy Parry got sued and lost, and it's software application is called Structure, Sequence, and Organization (SSO). You can in fact claim copyright on all characters that look like Harry Potter, because the standard for copyright infringement is "access and substantial similarity". This is because if your copyright doesn't extend to someone blatantly tracing over your work, then it's not a copyright.
You specifically need to argue that the API itself - the specific combination of types in a specific order, with a given set of Unicode or ASCII characters to identify it - is not copyrightable, not just that it's made up of uncopyrightable things. This is harder, because this same practice in other contexts (e.g. music, literature, and so on) is very much protectable. You need to argue that software is different.
That's a stretch. Oracle is claiming copyright on a specific API with a specific name, specific organization and specific individual components / attributes. It is not claiming copyright on all standard lib APIs. In fact, there is no evidence that Oracle has any intention whatsoever to sue Google over Go or Dart.
Now, the question is, where do we draw the line: As per Oracle, there cannot be any other character that does what Harry Potter does.