Would it be the classes Java has always had that sit inside a parent class and can access the instance fields/properties of the parent? These are very Java, and hard to recreate in a language that doesn't support that type of embedding. C# really struggles with it. Using the Android API via Xamarin was always very much harder to do anything with because of this symbiotic relationship. I have done a little Kotlin, but I don't remember if it support that or not.