[vbezhenar]

I don't know what you want to master with Java, it's pretty trivial language. So if you don't know something about Java, you definitely should learn it. The JVM and surrounding libraries are a thing, though. But Kotlin, Scala, Clojure for example all running on JVM. So you can learn new languages while learning JVM and improving your overall Java platform knowledge as well.

If you want to learn something, Scala is awesome. It's a very complex language, so learning it will make you a better programmer, even if you won't use it.

For practical purposes I like Kotlin. But Scala is good here too. Kotlin just clicked for me, I like almost everything about it.

Clojure is a good introduction into Lisp family. I think it's worth to learn it too.