I don't think there are any packages for talking to GPUs [At least there weren't the last time I read the big Java reference book], so it would only be good for multi-threaded CPU stuff.
There is jcuda
http://javagl.de/jcuda.org/I have seen some videos and examples, it looks promising, but I did not use it yet, as I decided to go to fully c project.
Apple provide Rosetta which could run x86_64 application[1]. Some benchmark on Apple M1 report it has decent performance[2], so why bother re-write Keyhunt code on java? IMO it's more practical to find way to compile Keyhunt on Mac environment instead.
It was not my intention, it was just a general remark about the solution which works on any platform.
Using pure java it is very difficult to have a performance comparable with a good c program. Let's forget about any ideas for rewriting keyhunt!