> And what is copied verbatim are just the headers

If you are using different languages that compile to a compatible target, or if you are using the same language that admits different literal declarations expressions that produce the same interface, it is possible to have an API reimplementation -- for the same target platform even -- that does not involve verbatim copying of even the headers/declarations.

It's possible in some languages, but it often involves serious messing around.

For example, in C, this would involve function name aliasing which is usually a compiler specific extension. This might give you name compatibility, but to get struct-level compatibility, you'd have to copy the exact layouts due to padding issues.

In C++ it's even harder, due to when vtables are emitted, etc.

My java knowledge is old (java 5), so i don't know if they made it possible.

Objective-C, I believe it would only be possible if you used the runtime reflection methods directly. I don't think you can easily do it with just headers (corrections welcome!) etc

I believe Java will look up methods etc by name (and signature) at runtime, so anything missing will just produce an exception when you try to use it.