How to avoid type mismatch error with kotlin generic?

Question

Code snippet

fun main() {
    val myClass : AbsClass<Any> = MyClass()
    // Error - type mismatch

    val myClass : AbsClass<*> = MyClass() 
    // Not Error, but parameter of 'foo' is 'Nothing'

    val myClass : AbsClass<Any> = MyClass() as AbsClass<Any> 
    // It's correct works with unchecked cast warning and unnecessary casting code.

    myClass.foo(MyModel())
}

class MyClass() : AbsClass<MyModel>() {
}

abstract class AbsClass<T> {
    fun foo(value:T){}
}

data class MyModel(val number:Int = 0)

Question

I want writing code as val myClass : AbsClass<Any> = MyClass() but, it shows type mismatch error. How to solve this problem(avoid type casting codeas AbsClass<Any>)?

I think this is not possible as foo has in semantics. This only works if you can change AbsClass<T> to AbsClass<out T> — Alexander Egger
– Alexander Egger, Commented Mar 5, 2019 at 7:12
Not familiar with Kotlin, but I believe generics in Kotlin is similar to what in Java, which is not covariant. i.e. Foo<Derived> is-not-a Foo<Parent>. So you cannot assign a AbsClass<MyModel> instance to a AbsClass<Any> reference. — Adrian Shum
– Adrian Shum, Commented Mar 5, 2019 at 8:09
Actually generics in Kotlin are covariant, you can read more about that here — functionaldude
– functionaldude, Commented Mar 5, 2019 at 15:34
@AlexanderEgger If you do that you can't use this fun foo(value:T){} IDE will complain saying Type parameter T is declared as 'out' but occurs in 'in' position in type T AbsClass<out T> — Farid
– Farid, Commented Sep 2, 2022 at 16:28

Cililing · Accepted Answer · 2019-03-08 20:18:53Z

7

You should use:

val myClass: AbsClass<out Any> = MyClass()` or `val myClass: AbsClass<in Nothing> = MyClass()

The out keyword means you want any subtype could be assigned to supertype. MyModel can by assigned to Any, so it's ok.

The in keyword means you expect any supertype could be assigned to subtype (in our case Nothing), so you can use your MyModel because it is the supertype of Nothing.

This way, without providing any information of expected type (you can specify, like AbsClass<T : MyAbstractModel>), in my opinion, there is really few possibilities without casting or reflection.

It's type variance and it's one of the most difficult ideas in OOP. More you can read here: https://kotlinlang.org/docs/reference/generics.html#variance

edited Mar 8, 2019 at 20:18

answered Mar 5, 2019 at 9:21

Cililing

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2 Comments

Farid Over a year ago

Don't know why OP accepted your answer but it is wrong one. In both case you will get Type mismatch. Required: Nothing, Found: MyModel when you try to call this myClass.foo(MyModel())

Cililing Over a year ago

Yep, but the question was about “how to declare”. That's why I have tried to explain later – using so wide type does not give a lot of possibilities.

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