[pdpi]

You have to write your Kotlin anticipating it'll be called from Java though (e.g. by using SAM interfaces instead of function types for your arguments)

[edem]

This does not mean that it is a one-way interop. When you write it the right way, it will work from Java.

[lmm]

Is Kotlin's function not an SAM interface? I'm pretty sure Scala's is.