when with enum

Learn branching on enums with when.

when with enum: exhaustive branches

When you list every constant of an enum in a when, no else is needed. If you add a new constant, the compiler demands a new branch; the error is caught early.

fun describe(c: Color) = when (c) {
    Color.RED -> "red"
    Color.GREEN -> "green"
    Color.BLUE -> "blue"
}

This is called an exhaustive when; every constant is written out. If you forget one, the compiler warns you. Adding a new constant is safe; every branch gets reviewed.

Exhaustion and new constants

If you add a new constant to an enum with an exhaustive when, the compiler warns you. This means adding a constant is safe.

enum class Color { RED, GREEN, BLUE }
// adding YELLOW breaks describe
// a new branch covering it is required

When you see the warning you add a branch; no constant slips through. This prevents future breakage. Writing every constant explicitly is safer than an else.

Exercise Describe directions

Define an enum class called Direction with NORTH, SOUTH, EAST, WEST. Write fun describe(d: Direction): String giving each direction a word with when. In main print describe(Direction.NORTH).

Show example solution

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

fun describe(d: Direction) = when (d) {
    Direction.NORTH -> "north"
    Direction.SOUTH -> "south"
    Direction.EAST -> "east"
    Direction.WEST -> "west"
}

fun main() {
    println(describe(Direction.NORTH))
}

Test yourself

If a when lists every enum constant, what is not needed?
- No else branch needed
- It needs a loop
- It only runs one branch
- It changes constants
Show answer

No else branch needed — Listing all constants means no else; the compiler enforces it.

Fill in the blank: add a when branch

when (c) {
    Color.RED -> "red"
    ____ -> "green"
}

Show answer

when (c) {
    Color.RED -> "red"
    Color.GREEN -> "green"
}

Match constant to text

Color.RED

red
Color.GREEN

green
Color.BLUE

blue

Next lesson: Enum and range

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