Question: Suppose I have this external interface I...

# getting-started

[Kelvin Chung]

Question: Suppose I have this external interface I have to implement:

interface MyInterface<T> {
  fun doSomething(): Array<T>
}

Is it possible to actually implement this interface generically?

class MyImpl<E> : MyInterface<E> {
  override fun doSomething(): Array<E> {
    // How do I create the array here?
  }
}

[Ronny Bräunlich]

That doesn't work unfortunately because the runtime wouldn't know what kind of array to create. Additionally, you cannot have a reified type parameter on a class.
What would work would be an extension function with a reified type parameter.

interface MyInterface {
}

inline fun <reified T> MyInterface.doSomething(): Array<T> {
    return emptyArray<T>()
}

class MyImpl : MyInterface {
}

fun main() {
    val myImpl = MyImpl()
    myImpl.doSomething<String>()
}

But in that case the implementation wouldn't be generic.

[Vampire]

And he cannot influence the interface, he said it is an external one

[ephemient]

to create generic arrays you either need reified types or

class MyImpl<E : Any>(val clazz: Class<E>)
java.lang.reflect.Array.newInstance(clazz, size)

[ephemient]

alternatively you could try to perform an unsafe unchecked cast from an Array<Any> - I think your function would "work", although I can envision scenarios in which the caller may end up performing a check itself and fail at runtime

[Kelvin Chung]

So, nothing within a KMP context, where java.lang.reflect.Array wouldn't be available? (I tried seeing if there was something in KClass that would allow me to create an array by reflection, but couldn't find anything)

[Youssef Shoaib [MOD]]

You could do this:

inline fun <reified E> myInterfaceImpl(): MyInterface<E> = object: MyInterface<E> {
  override fun doSomething(): Array<E> = emptyArray<E>()
}