So the main thing is libraries. If you write your library in Kotlin then it depends on the Kotlin runtime JARs which seriously bloats it's dependencies. Not a p

So the main thing is libraries. If you write your library in Kotlin then it depends on the Kotlin runtime JARs which seriously bloats it's dependencies. Not a problem where the library is only applicable in Kotlin-land but if it's generally useful from Java/Clojure/Scala then it's better to write it in Java. The other case is general OSS software, Java reaches a wider audience in my field (distributed databases, streaming data, etc). Java is pretty much considered the lingua-franca of Big Data with some very small Scala footprint and much less fluency in Kotlin. I generally write all my own stuff ontop of these on Kotlin but drop down into Java where I need to be able to share things.