In kotlin is there a way to give an interface to a...

# announcements

[Andrew]

In kotlin is there a way to give an interface to a class you don't own? I'm trying to reimplement a class from another library and want a shared contract between mine and that one as an intermediary until I'm finished.

[Colton Idle]

i dont believe so. The best thing you can do i think is to just wrap that class and have that wrapper implement your interface.

scenario
(Duck) Class I don't own
(Animal) Interface I created
(Cat) Class I own that impls Animal
Create a Duck2 class that implements the Animal, but is actually just a Duck class.

Then if you have List<Animal> you could add Cats and Duck2s, to your hearts content.

I think theres a name for that kind of pattern.. but im also new to kotlin so you can wait to hear from others too.

[jbnizet]

It's called "the adapter pattern". https://en.m.wikipedia.org/wiki/Adapter_pattern

[El Zhang]

Yep I agree with comments above.

open class A {
    fun method1() {
        println("A: method1")
    }
    fun method2() {
        println("A: method2")
    }
}

interface B {
    fun method2()
}

class C: A(), B

fun main() {
    val b: B = C()
    b.method2()
}

With this pattern you can even specify method2 as from B

[Rob Elliot]

It’s a shame kotlin won’t allow this:

class CannotBeChanged {
    fun someMethod() {}
}

interface NewInterface {
    fun someMethod()
}

class MyClass(delegate: CannotBeChanged) : NewInterface by delegate

In principal it could observe that CannotBeChanged meets all the requirements to implement NewInterface and so could be used as the delegate to do so in MyClass.

[Nir]

Yeah would be really nice for Kotlin delegation to be extended to handle more cases

[Andrew]

I went with a wrapper for the class but might change to extending the class + interface. Originally I did not to that because many methods contain the class itself but I can make the interface take a T parameter

[Hans Ellegard]

this is on many's wish list: https://youtrack.jetbrains.com/issue/KT-40900 and https://github.com/Kotlin/KEEP/pull/87
the last i heard was that it's being put aside for the moment, allowing development to focus on multiple receivers first. if you want the feature, you can upvote the KT issue.