Type constraints

Learn type constraints with where and upper bounds.

Type constraint

A type constraint forces a generic type parameter to be a subtype of a specific type. Kotlin uses T : UpperBound or the where keyword.

Example: fun <T : Comparable<T>> max(a: T, b: T): T { return if (a > b) a else b }. Here the T : Comparable<T> constraint requires T to be comparable. max(3, 7) works; max(listOf(1), listOf(2)) fails to compile. A type constraint ensures generic functions are called only with suitable types. This prevents runtime errors.

The max function

With a type constraint generic functions can use operators. The T : Comparable<T> constraint enables the > operator.

Example: fun <T : Comparable<T>> max(a: T, b: T): T { return if (a > b) a else b }. Calling max(3, 7) returns 7. Calling max("hello", "world") returns the alphabetically larger one. Thanks to the constraint, the > operator works for any comparable type. The where keyword can express multiple constraints.

Exercise The max generic function

Define the generic function fun <T : Comparable<T>> max(a: T, b: T): T whose body returns if (a > b) a else b. In main call println(max(3, 7)).

Show example solution

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

fun main() {
    println(max(3, 7))
}

Test yourself

What does the <code>T : Comparable<T></code> constraint provide in a generic function?
- That T is nullable
- That T is a subtype of Comparable<T>
- That T is Int
- That T can be any type
Show answer

That T is a subtype of Comparable<T> — The T : Comparable<T> constraint requires T to be a subtype of Comparable<T>.
Match the type constraint parts

<T : Comparable<T>>

Type constraint
fun max(a: T, b: T)

Function signature
: T

Return type
a > b

Comparison

Complete the max function constraint

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

Show answer

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

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