Kotlin Academy · Lesson

Exhaustive when

Cover all cases.

Lesson 4 of 413 steps

Exhaustive when is a free Kotlin Academy lesson on CoddyKit. This is lesson 4 of 4. Any 3 lessons of this learning path are free to read in full — after that, CoddyKit PRO unlocks every lesson, plus hands-on practice with a built-in code editor and a 24/7 AI tutor. It is part of the Kotlin Academy learning path, and your progress syncs across the web and the CoddyKit app. The Kotlin Academy course includes 4 lessons in total.

Covering All Cases

An exhaustive when handles every possible value. When used as an expression, Kotlin requires this so a result is always produced.

This lesson shows how to make when exhaustive and why it helps.

else Makes It Exhaustive

For open-ended subjects like Int, an else branch covers everything not listed.

This guarantees the when always matches.

fun label(n: Int): String = when (n) {
    0 -> "zero"
    1 -> "one"
    else -> "many"
}

fun main() {
    println(label(0))
    println(label(5))
}

Enums Enable Exhaustiveness

With an enum, the compiler knows every possible value. List them all and you may omit else.

enum class Direction { NORTH, SOUTH, EAST, WEST }

fun turn(d: Direction): String = when (d) {
    Direction.NORTH -> "up"
    Direction.SOUTH -> "down"
    Direction.EAST -> "right"
    Direction.WEST -> "left"
}

fun main() {
    println(turn(Direction.EAST))
}

Missing an Enum Case

If you forget an enum value in an expression when, the compiler reports an error.

This catches bugs the moment you add a new enum constant.

enum class Status { ACTIVE, PAUSED, STOPPED }

fun describe(s: Status): String = when (s) {
    Status.ACTIVE -> "running"
    Status.PAUSED -> "paused"
    Status.STOPPED -> "stopped"
}

fun main() {
    println(describe(Status.PAUSED))
}

Sealed Classes

A sealed class has a fixed set of subclasses known at compile time.

This lets when be exhaustive over all subtypes without else.

sealed class Shape
class Circle(val r: Double) : Shape()
class Square(val s: Double) : Shape()

fun area(shape: Shape): Double = when (shape) {
    is Circle -> 3.14159 * shape.r * shape.r
    is Square -> shape.s * shape.s
}

fun main() {
    println(area(Square(4.0)))
}

Why Exhaustiveness Helps

Exhaustive when turns missing cases into compile errors instead of silent bugs.

Add a new subclass or enum value and the compiler points to every when you must update.

Statements Are Not Forced

When used as a statement (not assigned), when does not require exhaustiveness.

But for enums and sealed types, being exhaustive anyway is good practice.

enum class Light { RED, GREEN }

fun main() {
    val light = Light.RED
    when (light) {
        Light.RED -> println("stop")
        Light.GREEN -> println("go")
    }
}

Combining Sealed and Data

Sealed hierarchies often use data class subclasses, giving you exhaustive matching plus smart casts to access properties.

sealed class Result
data class Ok(val value: Int) : Result()
data class Err(val message: String) : Result()

fun show(r: Result): String = when (r) {
    is Ok -> "ok: " + r.value
    is Err -> "error: " + r.message
}

fun main() {
    println(show(Ok(42)))
    println(show(Err("boom")))
}

else as a Safety Net

You can still add else to a sealed or enum when, but then the compiler stops warning about new cases.

Omitting else keeps the compile-time safety, so prefer that for closed types.

enum class Coin { HEADS, TAILS }

fun flip(c: Coin): Int = when (c) {
    Coin.HEADS -> 1
    Coin.TAILS -> 0
}

fun main() {
    println(flip(Coin.HEADS))
}

Boolean and Closed Subjects

A Boolean has only two values, so listing both makes when exhaustive without else.

fun yesNo(flag: Boolean): String = when (flag) {
    true -> "yes"
    false -> "no"
}

fun main() {
    println(yesNo(true))
    println(yesNo(false))
}

Putting It Together

Exhaustive when means all cases are covered:

else covers open subjects like Int
Enums, sealed classes, and Boolean can be exhaustive without else
The compiler flags missing cases when you add new ones

sealed class Event
object Start : Event()
object Stop : Event()

fun handle(e: Event): String = when (e) {
    is Start -> "starting"
    is Stop -> "stopping"
}

fun main() {
    println(handle(Start))
    println(handle(Stop))
}

Quick Check

Test your understanding of exhaustive when.

Answer it in the interactive lesson

Recap

You learned how to make when exhaustive:

else handles open subjects
Enums, sealed classes, and Boolean can be fully covered
Compile-time checks catch missing cases

That completes the When Expressions and Smart Casts course.

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Frequently Asked Questions

Is the “Exhaustive when” lesson free?

Yes — any 3 lessons of the Kotlin Academy learning path, including “Exhaustive when”, are free to read in full here on the web. After that, CoddyKit PRO unlocks every lesson, plus interactive practice with a built-in code editor and a 24/7 AI tutor. The Kotlin Academy course includes 4 lessons in total.

What will I learn in “Exhaustive when”?

Cover all cases. You practise Kotlin Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

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No prior experience is required. Kotlin Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 4 of 4.

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