Heat transfer to a pot is not an API, not even remotely close. It's a physical fact that has been observed since the stone age. The Java API is huge. It's not a mere physical fact, nor even a collection of method prototypes. It's everything covered in the JavaDocs: what happens in error scenarios, thread safety rules, how objects are combined to get different effects, what you can subclass and what's not, what security permissions things need. It's enormous and every decision in that was a 'creative' decision, in some sense.
From a plain reading of copyright law it's obvious APIs far smaller and simpler than Java should fall under copyright. The idea APIs don't is a convenient consensus-quo for the software industry that benefits competition and should probably be preserved, although there is the counterbalancing factor that the non-protectability of APIs probably has invisible side effects we can't see (e.g. under-investment in API design). But the Supreme Court won't or shouldn't care about any of that because it's a political debate. The question before them is about what the current law says, and on that, Oracle probably have a point. Also, it's going to be even more fashionable to hate on Oracle if they do win, but the current situation has always been legally fragile. Sooner or later someone would have observed the apparent skew between the software industry's approach to copyright and what the law really says. We may be at least somewhat grateful that it's a fight between two well resourced firms and not e.g. a patent troll firm type of situation, where the defenders are threatened and have to pay up because they're too small to fight it.
