You can mix and match Java files and Kotlin files in the same project without a problem. Also, for any Java file you can ask IDEA's Kotlin plugin to automatically convert it to Kotlin for you, and likewise if you cut-and-paste Java code into a Kotlin file it will ask if you want to rewrite that code block.
Kotlin's tooling for Java interop should go down in the textbooks as the prime standard for how to ease a programming language built for a run-time to mainstream adoption. It's just phenomenal.
Not that Kotlin is introducing anything like this, but, I wonder if this sets a precedent going forward for popular programming tooling vendors to evolve as the primary distribution point for introducing new programming language research and paradigms to the industry.
