@adibfara sure. First, user-side variance (projections):

val arr: Array<in Int> = arrayOf(1, 2, 3)

In that line you pledge to use only

set

-ing methods like

arr[0] = 1

, but not

get

-ing methods, e.i.

val x = arr[0]

will not compile

val arr: Array<out Int> = arrayOf(1, 2, 3)

Here it is the other way around, you restrict yourself to only use

get

and alike. Declaration-side variance is the same, but you declare from the start what way you can use a type. For example:

class MyList<in T> {...}

That declares that all methods of this class can only intake

T

, but not return. Meaning that

fun foo(): T

can NOT be a method in this class. It is similar for

out

. You can declare variance for each generic type:

class MyList<in T, out R> {...}

. You are free to mix and match variance however you like, for classes and functions, on declaration and user side.