[AnimalMuppet]

Disclaimer: I don't have a legal background.

If I understand correctly, Oracle would have to prove that Google's use of the Java API did not fall under the fair use exception in copyright law.

That is, if I write something, you can't publish it as your own. That's copyright. But there are exceptions within the law. One exception is fair use (others are criticism, parody, and there may be more, but fair use is the one in play in this case). Note that fair use was in the original trial, and the jury deadlocked on whether that was a valid defense for Google.

Below here, my understanding is less solid. But I think that one category of fair use is interoperability. That is, if I have a copyright interface (like, say, an API), you can copy that API in order to interface to something. To what degree, and what the boundaries are, I'm not sure.