[kelnos]

I don't know if the JVM developers have actually implemented this, but since records are immutable, there's no reason why a copy couldn't share memory between the instances.

The major downside is that could ruin cache locality and make passing the instance across a FFI boundary require a copy.

Another optimization would be that the JIT (or even perhaps javac) could notice cases where you make a copy (with one field changed) of a record and then never use the original reference again. If the JIT (or javac) can prove that no other bit of code holds a reference to the original record, it can reuse and mutate the original one instead of making a copy. I don't know if this optimization is or will be implemented, of course.

Either way, I expect the overhead you mention ends up being worth the benefits of immutable data. (That's been my experience using Scala, anyway.)