[nostrademons]

Hasn't really been my experience - I've found it pretty easy to write eg. Hadoop classes, Rome plugins, servlets from Kotlin. You do need to be aware of what code Kotlin is generating for you, and there are occasional warts, but in general you just implement your interfaces and enjoy the boilerplate that Kotlin frees you from typing.

[danneu]

I don't have interop issues with your examples either. Calling Java from Kotlin is very pleasant.

What I found is that it's the other direction that takes some thought. For example, my Kotlin library needs more help than just @Jvm annotations to be usable from Java.

[nostrademons]

All my examples are calls into Kotlin from Java. With Hadoop, you're implementing interfaces that the framework instantiates via reflection and then the Java framework calls the Kotlin code. With Rome, you're creating Kotlin objects that implement a Java interface and then they're called from Java code. With servlets, the servlet container (Jetty, in this case) holds a reference to the servlet and then calls its serve() method upon an HTTP request.