Is it possible to extend a singleton (object declaration) in Kotlin?

Question

I have an object declaration (object A : BaseClass) and I want to create another object declaration B that inherits from A's implicit class. BaseClass is a class and not an interface. Is it possible in Kotlin?

Answer 1

You can use delegation. Note that you can only delegate via interface, so you'll need to create an interface with subset of methods you need to delegate through.

interface Iface {
   fun doStuff()
}

object BaseObject: Iface {
   override fun doStuff() { }
}

class BaseClass: Iface {
   override fun doStuff() { }
}

object ExtendedObjectDelegatingToObject: Iface by BaseObject {
   fun doSomethingElse() { }
}

object ExtendedObjectDelegatingToClass: Iface by BaseClass() {
   fun doSomethingElse() { }
}

class ExtendedClassDelegatingToObject: Iface by BaseObject {
   fun doSomethingElse() { }
}

class ExtendedClassDelegatingToClass: Iface by BaseClass() {
   fun doSomethingElse() { }
}

fun example() {
    BaseObject.doStuff() // can call doStuff from BaseObject
    ExtendedObjectDelegatingToObject.doStuff() // can call doStuff from ExtendedObject
    ExtendedObjectDelegatingToObject.doSomethingElse() // can also call additional methods on ExtendedObject
}