Kotlin is an open source programming language from Jetbrains that can use existing Java or JavaScript frameworks. In this talk Travis Spencer explains how to use Kotlin in an existing Java codebase.
More information on this topic on http://nordicapis.com/building-apis-on-the-jvm-using-kotlin-and-spark-part-1/ and http://nordicapis.com/building-apis-on-the-jvm-using-kotlin-and-spark-part-2/