Nothing's stopping you from making a Transferable interface. Everything that implements it has to implement transfer(), and you can then make an extension funct

Nothing's stopping you from making a Transferable interface. Everything that implements it has to implement transfer(), and you can then make an extension function on Transferable instead of your implementation. Additionally, multiple dispatch in JVM languages (except Groovy because it implements multiple dispatch already) can just be done by having two layers of classes. Yes, it's a holdover from the JVM being heavily class based, but nothing preventing you from doing that. Hell, define both your classes, and put all your logic in a single file with extension functions. Kotlin is quite free form when it comes to where you should put your things, what it should be attached to, etc.