Yes there is a Java api to Cocoa. But the current implementation is in C++. There is an ObjectiveC++ compiler (compiles a mix of ObjectiveC and C++ code making bridging/wrapping easy). Plus, the Java apis to Cocoa are weak compared to the ObjectiveC ones *and* the JVM imposes a VISIBLE performance penalty in both size and speed.
So why saddle the port with Java at all?
Dumb dumb FUCKING dumb idea.