why can i do

val x: List<Int> = listOf(1, 2, 3)
val y: List<Number> = x //works fine

but not

val x: Array<Int> = arrayOf(1, 2, 3)
val y: Array<Number> = x //error: type mismatch

?

This is due to the generic variance: https://kotlinlang.org/docs/generics.html#variance List is defined as

List<out E>

and Array as

Array<T>

. In your example list can only return

Int

elements. So this allows the cast to

Number

since every

Int

is also a

Number

. Array however also accepts elements (e.g. in the

set

function). Since it expects an

Int

in these functions you won't be able to cast the array to

Number

. This is because while all `Int`s are `Number`s not all `Number`s are `Int`s.

Array however also accepts elements

i dont understand this part, what does

element

mean exactly?

With element I mean an item in the array or list. So in your example

1

,

2

and

3

are the elements.

but then what is the sentence

Array however also accepts elements

supposed to mean? it kinda implies that lists dont accept elements which makes no sense

Yeah a

List

is immutable. So you can't add more elements to it (if you want to add elements to a list you would need a

MutableList

).

Array

however does allow you to update the elements with the

set

function.

oooh thats what you meant. got it ty