Oh yeah - the seamless Java/kotlin interop is pretty slick, though there are some pitfalls - nullity is one of them. Kotlin treats it explicitly, but when it’s talking to Java, it’s forced to treat it ambiguously unless you wrap stuff in Optional. And then even if you do, some Java lib that you need to plug into your project might not do that, so you’ve got to be careful about managing the interactions between their interfaces in some situations. And then there’s the unfortunate divergence that’s started to occur as newer versions of Java have brought certain features that were first available in Kotlin into Java, but the patterns and interfaces often differ subtly, so you have to worry about that too, especially with less experienced engineers on the team.

Don’t get me wrong - it’s great, and I’ve enjoyed working with it, but if you’re doing an incremental migration, make sure you test the ever-loving shit out of it.

Source: I did an incremental Java -> Kotlin migration on a huge project a couple jobs ago