Sure, and Scala has had ADTs since its inception as well I think, and that's also JVM. It's not ADTs, but Clojure does have some level of pattern matching/destructuring as well.

It wasn't that I though that the JVM was incapable of doing something like an ADT, just that vanilla Java didn't support it. While it's easy to say that "companies should just use Kotlin", that's a bit of a big ordeal if you already have a 15 year old codebase that's written in Java.

I've heard of but never used the Functional Java library, though it'd be a tough sell to get my work to let me import a library that hasn't been updated in two years.