It generally is. That's the whole point of clean-room reimplementation. You have one set of engineers that reverse engineers a product to develop a spec, and another set that implements the same functionality from the spec. That's kosher and has been upheld in several court cases.

The trouble for doing that with Java APIs is the spec basically is the class declarations. You don't just need to "do the same thing" you need a tangible source file that has at least in part the same text.