Preview Feature in Java 21 — The Vector API has been in preview since Java 16 (JEP 338). JEP 448 is the sixth preview iteration in Java 21. The API is stable and production-usable with --enable-preview; finalization is pending Project Valhalla value types. What Is SIMD and Why Does It Matter? Modern CPUs can perform the same arithmetic operation on multiple data values in a single instruction. This is called SIMD — Single Instruction, Multiple Data.