> genuine breakthrough
Well, it is - because they had to make it with almost perfect backwards compatibility for one of the most popular languages with trillions of lines of code produced over decades.
Sure, adding it to a new language is not hard. Adding it to Java which has primitives, generics and boxing, finding ways that seamlessly cover the differences between objects and primitives, while trying to plan for the future is hard.
As a general note, if you come to the conclusion that one of the best designer teams on Earth "basically copied what .NET did from year 1 is not a good look", then maybe your mental model needs adjusting on how these stuff works? Java has a public mailing list, you can browse through the related discussions. Implementation is the least of these things. But I can assure you they most definitely know what they are doing.
> because they had to make it with almost perfect backwards compatibility for one of the most popular languages with trillions of lines of code produced over decades.
At what cost? A key benefit of value types is improved performance but AFAIK Valhalla doesn't even let you pass them by reference. Efficiently passing them through registers is great but won't help you out with larger value types.
idk maybe java should adopt something similar to rust's "edition"?
Correct me if I'm wrong, but Rust editions are a source code-level feature. So given you have the source code of newer and older rust code, you can compile them together.
That's materially distinct from Java's model of basically dynamic loading already compiled class files. Though class files do have "editions", and there are extra code to deal with different versions. But still, it should be possible to e.g. send a new value class to an old library's class that has never heard of them, and that should just work.
The important thing is that Rust editions affect semantics and name resolution. In such an analogy, JVM bytecode is the equivalent of Rust code - various semantics are baked in, but stuff like name resolution isn't (at least not completely).
Except of course they are breaking backwards compatibility, in relatively subtle ways, for anything that uses the standard library wrapper types. So every use of Integer, Boolean , etc is an opportunity for either a compilation error or a runtime bug.