It has very smooth interop, both languages can call each other's methods with no fuss. You can mix and match kotlin and Java files in the same directory. There

It has very smooth interop, both languages can call each other's methods with no fuss. You can mix and match kotlin and Java files in the same directory. There are one-click tools for auto converting Java to kotlin. It is probably the main reason it beat out Scala on Android, along with the small runtime and superior IDE support.