What is variance in Kotlin generics?

Variance controls how generic types relate when their type parameters are related. out T means the type is a producer (covariant) — you can assign Box to Box . in T means the type is a consumer (contravariant). Use-site and declaration-site variance are both supported in Kotlin.

Why do we need generics instead of Any?

Generics preserve type safety at compile time. List cannot accidentally receive an Int, and you do not need casts when reading elements.

Intermediate 8 min readKotlin 2.0

Kotlin Generics — Type-Safe Reusable Code

Generics allow you to write functions and classes that work with any type while preserving type safety at compile time.

What You Will Learn

Generic functions with <T>
Generic classes
Type bounds with where <T : Type>
Variance: out (covariance) and in (contravariance)

Generic Functions

Add <T> before the function name to make it generic.

Generic Function

kotlin
fun <T> printTwice(value: T) {
    println(value)
    println(value)
}

fun <T> firstOrNull(list: List<T>): T? {
    return if (list.isEmpty()) null else list[0]
}

fun main() {
    printTwice("Hello")
    printTwice(42)
    println(firstOrNull(listOf(10, 20, 30)))
    println(firstOrNull<String>(emptyList()))
}

Output

Hello Hello 42 42 10 null

<T> declares a type parameter. T can be any type — the compiler infers it from the argument. firstOrNull returns T? — the type or null.

Beginner Tip: The Kotlin standard library uses generics extensively — List<T>, Map<K,V>, and Optional<T> are all generic types.

Generic Classes

Classes can be generic too.

Generic Class

kotlin
class Box<T>(val value: T) {
    fun describe() = "Box containing: $value (${value!!::class.simpleName})"
}

fun main() {
    val intBox = Box(42)
    val strBox = Box("Kotlin")
    println(intBox.describe())
    println(strBox.describe())
}

Output

Box containing: 42 (Int) Box containing: Kotlin (String)

Box<T> is a generic class. Each instance fixes T to a specific type. intBox is Box<Int>; strBox is Box<String>.

Type Bounds

Constrain T with an upper bound using : Type.

Type Bounds

kotlin
fun <T : Comparable<T>> max(a: T, b: T): T {
    return if (a > b) a else b
}

fun main() {
    println(max(3, 7))
    println(max("Apple", "Banana"))
    println(max(3.14, 2.71))
}

Output

7 Banana 3.14

T : Comparable<T> means T must implement Comparable. This allows using > inside the function. Works with Int, String, Double, and any Comparable type.

Practice Exercise

Exercisemultiple choice

What does <T : Number> mean in a generic function?

Quick Quiz

What keyword declares a type parameter in Kotlin?

Kotlin Generics — Type-Safe Reusable Code

What You Will Learn

Generic Functions

Generic Function

Generic Classes

Generic Class

Type Bounds

Type Bounds

Practice Exercise

Quick Quiz

Frequently Asked Questions

Related Tutorials