Former Groovy dev here. I read pretty much all the comments here and the thing I am struck by is that we had pretty much everything Kotlin has in like 2011 with Groovy, it's just that for some reason a lot of assholes either didn't like the name or "muh strong typing" was a big deal, iirc.

Anyhow, my current team wants to move to Kotlin for Spring Boot 2 microservice work. I'm not against it per se, and I spent a lot of time learning the language and all its cool features. The issue I have isn't the language but rather that:

1) I would have to split my time between Kotlin and Java and I would like it to be one or the other. I already did that with Groovy and my Java suffered for it, and so did I. It's like losing your magic Jedi abilities to go back to Java.

2) I have developers who can't successfully write good Java, and looking at Kotlin, it is far more sophisticated and easy to blow one's foot off with. No thanks. I have enough code reviews. Several people I work with can't do Java streams correctly.

3) Recruitment for Kotlin devs is impossible. You have to train people and companies won't do that, so there's no pool of talent ready to go.

4) Future job changes--see #1. I spent a ton of time doing Groovy (okay, and Clojure) only to have those rugs completely pulled out from under me later on. I'm flexible so whatever, but like I said, it was like losing my Jedi powers.