[zmmmmm]

> Kontlin/Scala/Clojure all have relatively easy Java-interop

I think that you need to separate out languages that are intended for bidirectional interop (Kotlin/Groovy) from those that provide excellent one-way interop (language uses Java APIs) but less optimal interop the other way (Java uses code written natively in language).

Groovy and Kotlin have great bidirectional interop while Clojure and Scala don't map over nearly as well.

> it smells like after a while non-Java projects could just become Java projects?

Almost never happens in my experience, Java is just too suboptimal compared to these other languages. Once I started using Groovy in a few places it quickly becomes the case that I wanted to use it absolutely everywhere I can - it's just too much better.

I do code it in a much more static style than is often used in Groovy more generally though (declaring types whenever it helps readability, etc). I also always want to define interfaces and key data structures in pure Java if there's no reason to have them written in a different language.