Struggling to understand why the overriding works ...

# announcements

[MrNiamh]

Struggling to understand why the overriding works with val but not with var. What changes with var?
This works:

class A (
    override val values: List<A>
) : B

interface B {
    val values: List<B>
}

This does not:

class A (
    override var values: List<A>
) : B

interface B {
    var values: List<B>
}

[lhwdev]

Assume this code does work.
In the second one, you can put values from some value with type `List<B>`:
for example,

class C : B { ... }
val a: A = A(...)
val b: B = a
b.values = listOf<B>(C())

This becomes available, then when getting from `a.values`(which is just alias to b.values with different type), values is expected to be instance of List<A>; in fact it isn't.

a.values[0] // not the instance of A

[MrNiamh]

Ah, I see. That makes sense, thank you!

[Jilles Soeters]

I would have never guessed that. Great explanation @lhwdev

[rnett]

See also: https://kotlinlang.org/docs/generics.html#variance