Overriding generic function using sub-class type parameter

Question

I have the following interface in kotlin, not modifiable because it's defined in a third party library. And I would like to override the generic method using a type parameter of the subclass:

interface Factory {
    fun <T> create(modelClass: Class<T>): T
}


// The naive solution doesn't compile :  "Class is not abstract and does not implement abstract member":
class ConcreteFactory1 <T> (val creator: () -> T) : Factory {
    override fun create(modelClass: Class<T>): T {
        return creator()
    }
}


// If we introduce a second type parameter, it compiles, but generates a "Unchecked Cast" warning:
class ConcreteFactory2 <T> (val creator: () -> T) : Factory {
    override fun <T2> create(modelClass: Class<T2>): T2 {
        return creator() as T2
    }
}

Is there a way to achieve this without compilation warning ?

al3c · Accepted Answer · 2020-05-15 12:42:00Z

1

It depends. That interface requires that you're able to handle any possbile Class<T> and your implementation doesn't.

So the question is what do you want to do when create is called with a type which is not the one your ConcreteFactory2 can create?

If the only thing you want is not to see the unchecked cast you can do this:

class ConcreteFactory2 <T> (val creator: () -> T) : Factory {
    override fun <T2> create(modelClass: Class<T2>): T2 {
        return creator() as? T2 ?: throw IllegalArgumentException("unsupported type")
    }
}

But whether that is an acceptable implementation of that interface really depends on how is that used.

answered May 15, 2020 at 12:42

al3c

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4 Comments

Yann Armelin Over a year ago

Warning is still there with this solution Unchecked cast: T to T2

Tenfour04 Over a year ago

Since the type isn't reified, I don't think a safe cast operator will work. You'll just get a ClassCastException when it returns instead of the more informative IllegalArgumentException. But this approach could work if you have a KClass property in the constructor so the type can be reified and checked for real.

Tenfour04 Over a year ago

I'm wrong. That would only reify T, not T2. There's no way to reify T2, so you can't safely cast it.

al3c Over a year ago

whoops, sorry didn't try that out

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