Kotlin - Problem extending a generic class and returning a subtype

Question

I have the following situation:

• An abstract generic class

abstract class K<T> {
  abstract fun bind(itemModel: T)
}

• A class that extends from K

class A : K<ModelA> {
    override fun bind(itemModel: ModelA) {
        // ... do anything
    }
}

class B : K<ModelB> {
    override fun bind(itemModel: ModelB) {
        // ... do anything
    }
}

• Another class, this is a factory that returns a subtype of K based on some parameter:

class Factory {
    companion object {
        const val TYPE_A: Int = 0
        const val TYPE_B: Int = 1

        fun create(type: Int): K {
            return when (type) {
                TYPE_A -> A()
                TYPE_B -> B()
            }
        }
    }
}

At this point, I'm getting an error in return type section of Factory.create method, it says: "One type argument expected for class K< T >". It's clear that I'm trying to return a subtype of K in create method, and it's the only thing that matters, not the specific type of a generic.

This is possible in Java. What is the correct way in Kotlin?

Answer 1

There are multiple things to improve here (updated according to the comment).

• You need to add default value if the parameter in not TYPE_A or TYPE_B
• Add * to parametrized K return type.

As a result it will be something like that:

fun create(type: Int): K<*> {
   return when (type) {
       TYPE_A -> A()
       TYPE_B -> B()
       else -> throw IllegalStateException("useful message")
    }
}