Yeah but if you're appending in a loop you'll need to allocate memory every time, right? Can a JIT avoid or rewrite this?

Yep the Hotspot JIT will absolutely allocate on the stack and reuse memory allocations inside loops. It also has escape analysis and it will detect you don't need the array of strings after the iteration. In that case it usually just allocates the strings directly at their final locations on the record object.

Wow, that's super cool. Almost makes me wish i did stuff on the JVM (but until someone rewrites LAPACK for it, that seems unlikely).