hi there. is there a way to apply an interface to an existing object? for example, i might like to m...

# getting-started

[Jonathan Locke]

hi there. is there a way to apply an interface to an existing object? for example, i might like to make an Addable interface and apply it to Collection, sort of like extension methods, except extension interfaces. thanks

[August Lilleaas]

as in, make built-in interfaces/classes implement your own interfaces?

[Jonathan Locke]

yes. something like Collection : Addable, where Addable is my interface.

[Jonathan Locke]

where Addable is:

interface Addable { 
    fun add(element: E): Boolean 
}

[Vampire]

But how should an instance of Collection then implement that interface?
Or do you only use that to redeclare methods that are already present?
If the latter, then you probably wait for denotable union and intersection types I'd guess.

[Jonathan Locke]

only what is already implemented. so List implements Collection.add(E) and I could pass a list instance to a method that takes Addable. is there a place I can read about union and intersection types?

[Jonathan Locke]

if you mean that I could write a method like myMethod(container : Addable | Collection), it's not really the same thing because you would have to add an arbitrary number of types to the union. but maybe I'm misunderstanding?

[Jonathan Locke]

that's also useful, of course, but what i'm asking about is more like

interface kotlin.collections.Collection.Addable

(consistent with extension method syntax

[Vampire]

I don't think it is possible, and I'd doubt it will ever be possible.
Probably the closest is to have an extension method or extension property toAddable or asAddable or similar that gives you an Addable wrapper for that type.

[Jonathan Locke]

I guess it could be too complicated

[Jonathan Locke]

thanks

[Jonathan Locke]

the extension method is a good idea as a substitute