Java uses multicores... I am pretty sure when you launch minecraft you can use java arguments like -XX: ParallelGCThreads=8 This will let java use more then one core to process the game. I notice a pretty big speed increase when i do this.
No, this will let the garbage collector use more threads, that's it. The program itself has to be written to take advantage of multicore CPUs. Also note that multiple threads and multiple cores are not synonymous in programming. I can write a multithreaded program that only runs on one processor.