Is it possible to add an interface to an existing class in Kotlin?

Question

Sometimes it is useful to create an interface that is already implemented by existing third-party code (ie. from a library). For example, we could imagine selecting a subset of the methods of the String class and declaring a GenericString. We might then define other GenericStrings that implement these methods, but not other methods of the String class. The only problem would be that the String class doesn't inherit from the GenericString class. Is it possible to add an interface to an existing class in Kotlin or would I have to create a sub-class of String that implements this interface?

Answer 1

No, unfortunately "extension types" are not supported. There was a discussion about this on the Kotlin forums (here), but there is nothing similar that is already in the language.

There is a KEEP for this if you would like to vote.

One possible workaround, although it is not fully equivalent, is to have an interface representing your desired behavior and create an extension method returning an implementation ("delegate") for every type you want to use:

fun String.toGenericString() = object : GenericString {
    private val str = this@toGenericString
    override val length get() = str.length
    //...etc
}

and then use this instead.