Having led engineering at a startup that developed ~95% of our code in Scala and then moving to a FANG team that developed most of its services in Java (still u

Having led engineering at a startup that developed ~95% of our code in Scala and then moving to a FANG team that developed most of its services in Java (still using Spark Scala to a large degree and beginning to introduce Kotlin), these are the reasons I can use to justify using Java over either: 1) it is already there, 2) most engineers know Java well [enough], 3) other teams predominantly use Java and it makes certain types of code sharing more streamlined. I'm assuming most Java shops have existing tooling that will lock them into the JVM no matter what. That's fine, the JVM is incredible. Kotlin can be introduced very easily into most Java codebases as interoperability is a tenet. Scala not so much. Holistic interop between Scala and Java can fill an entire intermediate-level book. At the startup, we had a difficult time hiring for Scala. A number of candidates we moved forward with who were well-versed in it wanted to explore the language and embrace the FP aspects, which led to incomprehensible code for the engineers used to procedural OOO. In full retrospect, this was an issue with our hiring standard; Scala should not have been a specific part of our bar. It was a startup, we had crushing customer demands, there was no time to teach every engineer what an IO Monad was. It was a constant battle and perhaps the most frustrating aspect to me leading all the way up to acquisition. None of these issues would have existed if we chose e.g. Go; we would have quickly found a team of engineers who wanted to build applications instead of tinkering with the language. To me, that's the best part about Go -- it's boring. Java is another boring language, but it's also a very verbose one, even when fully utilizing Lombok/G-libraries/etc.. Kotlin fixes the verbosity issue to a large enough degree for my needs without introducing enough interesting features to attract those darn academic types. To me, Kotlin is about as good as a language can get while remaining on the current iteration of the JVM. We have been slowly introducing Kotlin and plan to use it for most new non-Spark-related projects. I would only consider Scala for the same purpose if 1) interop was trivial to the degree that Kotlin's is, and 2) it was reduced to a reasonable subset of its full complexity. I believe the right steps are being taken with the language, but it still has a ways to go in those regards.