Enum properties

Learn to define properties and methods in enums.

Enum properties: constructor and field

An enum class can take a constructor and properties. Each constant is declared with a value that fits the constructor.

enum class Planet(val distance: Int) {
    MERCURY(57), VENUS(108), EARTH(150)
}
println(Planet.EARTH.distance)  // 150

Each constant takes a value in parentheses; the value binds to the distance property. This lets you attach meaningful data to constants. Data lives with the constant; no separate table is needed.

Methods inside an enum

You can define a method inside an enum class. The method can access the constant's properties.

enum class Planet(val distance: Int) {
    MERCURY(57), VENUS(108), EARTH(150);
    fun isClose() = distance < 100
}
println(Planet.EARTH.isClose())  // false

A semicolon goes after the constant list; then the method follows. Inside the method you access properties like distance directly. This way behaviour lives with the data.

Exercise Add an emoji property to Mood

Define an enum class called Mood with a constructor property val emoji: String. Constants: HAPPY(":)"), SAD(":("). In main print the emoji value of the HAPPY constant.

Show example solution

enum class Mood(val emoji: String) {
    HAPPY(":)"), SAD(":(")
}

fun main() {
    println(Mood.HAPPY.emoji)
}

Test yourself

Fill in the blank: declare the property

enum class Planet(val ____: Int) { MERCURY(57), VENUS(108) }

Show answer

enum class Planet(val distance: Int) { MERCURY(57), VENUS(108) }

What does enum class Planet(val distance: Int) define?
- An interface
- Only a method
- Only constants
- A property and constructor
Show answer

A property and constructor — The distance property comes from the constructor parameter.
Match piece to role

distance

Planet property
MERCURY(57)

Constructor call
isClose()

Method
57

Value

Fill in the blank: call the method

val p = Planet.EARTH
val close = p.____()

Show answer

val p = Planet.EARTH
val close = p.isClose()

Next lesson: when with enum

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