News! Google is adopting Java8 for Android N alongside their new “Jack” compiler infrastructure. This raises an obvious set of questions for how Kotlin will interoperate with this (e.g., how Kotlin lambdas can be passed to Java8 and vice versa). My observation playing with this a while ago, with vanilla Java8 were that Java8 lambdas and Kotlin lambdas don’t have the same type, which can get a bit klunky.
I haven’t dug through the new Android N APIs yet, but I’m assuming that they’re still built with old-Java (so no default methods, no lambdas), but in the next year or two, you can expect that to start changing.