JVM languages those are easy and worth to learn:
- Kotlin. It will take you only a few hours to get started and it will make you much more productive.
- Groovy. Nice scripting language very good for simpler tasks and prototyping. Very handy when you need to do more advanced stuff with ant or gradle.
JVM languages those are not easy but well worth the effort:
- Scala. Very powerful. Code becomes about half of equivalent Java code and encourages you to start thinking about programming in new ways. Play Framework makes enterprise application development fun and exciting again. An alternative to learning Scala would be to look at Clojure.
- R. The implementation of R for the JVM allows you to take data analysis to a level plain Java cannot while at the same time easily integrate your R code with any JVM based eco system, e.g. integrating with Play Framework or Spring Boot applications.