So I could write a "clean-room" re-implementation of e.g. the entire Java environment and calls and sell that and Oracle is just gonna sit there and do nothing? They're surely gonna do _something_!
Google was sued for doing something similar but they only re-implemented a small amount of the API. They "got away with it" for the moment because a jury finally decided the particulars of what they did fell under fair use.
"A court initially found that Google did not violate Oracle?s copyright, but the decision was overturned when an appeals court found that the APIs in question are indeed copyrightable." Then that decision was overturned on a third appeal. Oracle naturally said they would appeal again.
I think if these legal questions were as "Clearly you don't understand..." as you make them out to be nobody would ever be sued but, clearly, it happens.