Variance

Learn in, out and variance concepts.

Variance

Variance describes the subtype relation between generic types. Kotlin uses out (covariance) and in (contravariance) keywords.

Example: interface Producer<out T> { fun produce(): T }. out T means T is used only in an producing (out) position. Producer<Int> becomes a subtype of Producer<Number>. in T means T is used only in a consuming (in) position. Variance gives flexibility in generic APIs.

out vs in

out and in describe opposite directions. out T only produces the type; in T only consumes it.

Example: interface Producer<out T> { fun produce(): T }; interface Consumer<in T> { fun consume(t: T) }. Producer<Int> can be used as a Producer<Number> (Int is a subtype of Number). Consumer<Number> can be used as a Consumer<Int>. Variance makes generic types fit the subtype hierarchy. This adds flexibility in function parameters.

Exercise The Producer<T> generic interface

Define an interface interface Producer<out T> with a fun produce(): T method. Define a class class IntProducer(val value: Int) : Producer<Int> with override fun produce(): Int = value. In main declare val p: Producer<Number> = IntProducer(42) and print the result with println(p.produce()).

Show example solution

interface Producer<out T> {
    fun produce(): T
}

class IntProducer(val value: Int) : Producer<Int> {
    override fun produce(): Int = value
}

fun main() {
    val p: Producer<Number> = IntProducer(42)
    println(p.produce())
}

Test yourself

Complete the out variance definition

interface Producer<____ T> {
    fun produce(): T
}

Show answer

interface Producer<out T> {
    fun produce(): T
}

Match the variance parts

out T

Producer (covariance)
in T

Consumer (contravariance)
fun produce(): T

Producing method
fun consume(t: T)

Consuming method
What does <code>out</code> specify in <code>interface Producer<out T></code>?
- That T is only produced
- That T is only consumed
- That T is mutable
- That T is nullable
Show answer

That T is only produced — out T means T is only produced.

Next lesson: Type constraints

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