What is out keyword in kotlin

Question

I am not able to understand and I couldn\'t find the meaning of out keyword in kotlin.

You can check example here:

List
         


        

Answer 1

With this signature:

List

you can do this:

val doubleList: List = listOf(1.0, 2.0)
val numberList: List = doubleList

which means T is covariant:

when a type parameter T of a class C is declared out, C can safely be a supertype of C.

This is contrast with in, e.g.

Comparable

you can do this:

fun foo(numberComparable: Comparable) {
  val doubleComparable: Comparable = numberComparable
  // ...
}

which means T is contravariant:

when a type parameter T of a class C is declared in, C can safely be a supertype of C.

Another way to remember it:

Consumer in, Producer out.

see Kotlin Generics Variance

-----------------updated on 4 Jan 2019-----------------

For the "Consumer in, Producer out", we only read from Producer - call method to get result of type T; and only write to Consumer - call method by passing in parameter of type T.

In the example for List, it is obvious that we can do this:

val n1: Number = numberList[0]
val n2: Number = doubleList[0]

So it is safe to provide List when List is expected, hence List is super type of List, but not vice versa.

In the example for Comparable:

val double: Double = 1.0
doubleComparable.compareTo(double)
numberComparable.compareTo(double)

So it is safe to provide Comparable when Comparable is expected, hence Comparable is super type of Comparable, but not vice versa.