I'm sold on Kotlin and have been working professionally in it for a few years (mixed Kotlin/Java projects server-side), so take my opinion for what it's worth.

I'm sold on Kotlin and have been working professionally in it for a few years (mixed Kotlin/Java projects server-side), so take my opinion for what it's worth. Integrating with Java's existing build systems (Gradle, Maven) was one of JetBrains initial design goals to avoid the separate ecosystem requirement that initially came with Scala. I admit building Kotlin has some quirks, notably around annotation processors - but overall it is very seamless. I am happy Java is adding new stuff (records, switch expressions, sealed classes), but I worry it is increasing overall language complexity vs. the fresh start that Kotlin was able to take. Supporting the legacy patterns in the language adds to the complexity any given Java developer is supposed to know. Overall I think the sane defaults and nudges towards the right patterns Kotlin pushes are a much bigger deal than just reducing boilerplate - though that is nice too.