Kotlin Enum Classes

An enum class defines a fixed set of named constants. Use enums when a variable should only ever hold one of a specific, known set of values — like days of the week, directions, or status codes.

Basic Enum

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

fun main() {
    val heading = Direction.NORTH
    println(heading)        // NORTH
    println(heading.name)   // NORTH (as String)
    println(heading.ordinal) // 0 (index position)
}

Enum in when Expression

enum class Season { SPRING, SUMMER, AUTUMN, WINTER }

fun describe(s: Season): String = when (s) {
    Season.SPRING -> "Flowers bloom"
    Season.SUMMER -> "Hot and humid"
    Season.AUTUMN -> "Leaves fall"
    Season.WINTER -> "Cold and snowy"
}

fun main() {
    Season.values().forEach { println("${it.name}: ${describe(it)}") }
}

Output:

SPRING: Flowers bloom
SUMMER: Hot and humid
AUTUMN: Leaves fall
WINTER: Cold and snowy

Enum with Properties

enum class Planet(val radiusKm: Double, val massKg: Double) {
    MERCURY(2_440.0, 3.30e23),
    VENUS  (6_052.0, 4.87e24),
    EARTH  (6_371.0, 5.97e24),
    MARS   (3_390.0, 6.42e23);

    val gravity: Double
        get() = 9.8 * (massKg / 5.97e24) / ((radiusKm / 6_371.0) * (radiusKm / 6_371.0))
}

fun main() {
    Planet.values().forEach {
        println("${it.name}: gravity = ${"%.2f".format(it.gravity)} m/s²")
    }
}

Enum with Functions

enum class TrafficLight {
    RED {
        override fun action() = "Stop"
        override fun next() = YELLOW
    },
    YELLOW {
        override fun action() = "Slow down"
        override fun next() = GREEN
    },
    GREEN {
        override fun action() = "Go"
        override fun next() = RED
    };

    abstract fun action(): String
    abstract fun next(): TrafficLight
}

fun main() {
    var light = TrafficLight.RED
    repeat(6) {
        println("${light.name}: ${light.action()}")
        light = light.next()
    }
}

Output:

RED: Stop
YELLOW: Slow down
GREEN: Go
RED: Stop
YELLOW: Slow down
GREEN: Go

Enum Utility Functions

enum class Status { PENDING, ACTIVE, SUSPENDED, CLOSED }

// Get all values
println(Status.values().toList())
// [PENDING, ACTIVE, SUSPENDED, CLOSED]

// Get by name (case-sensitive)
val s = Status.valueOf("ACTIVE")
println(s)   // ACTIVE

// Safe lookup using enumValueOf
val safeStatus = runCatching { Status.valueOf("INVALID") }
    .getOrDefault(Status.PENDING)
println(safeStatus)  // PENDING

Practical Example: Order Status Tracker

enum class OrderStatus(val displayName: String, val isFinal: Boolean) {
    PLACED    ("Order Placed",   false),
    CONFIRMED ("Confirmed",      false),
    SHIPPED   ("Shipped",        false),
    DELIVERED ("Delivered",      true),
    CANCELLED ("Cancelled",      true);

    fun next(): OrderStatus? = when (this) {
        PLACED    -> CONFIRMED
        CONFIRMED -> SHIPPED
        SHIPPED   -> DELIVERED
        else      -> null   // final states have no next
    }
}

fun main() {
    var status = OrderStatus.PLACED
    while (status != null) {
        println("Status: ${status.displayName}")
        if (status.isFinal) break
        status = status.next() ?: break
    }
}

Output:

Status: Order Placed
Status: Confirmed
Status: Shipped
Status: Delivered