Enum Classes

Enum classes are used to model types that represent a finite set of distinct values, such as directions, states, modes, and so forth.

enum class State {
    IDLE, RUNNING, FINISHED                           // 1
}

fun main() {
    val state = State.RUNNING                         // 2
    val message = when (state) {                      // 3
        State.IDLE -> "It's idle"
        State.RUNNING -> "It's running"
        State.FINISHED -> "It's finished"
    }
    println(message)
}

1. Defines a simple enum class with three enum constants. The number of constants is always finite and they are all distinct.
2. Accesses an enum constant via the class name.
3. With enums, the compiler can infer if a when-expression is exhaustive so that you don't need the else-case.

Enums may contain properties and methods like other classes, separated from the list of enum constants by a semicolon.

enum class Color(val rgb: Int) {                      // 1
    RED(0xFF0000),                                    // 2
    GREEN(0x00FF00),
    BLUE(0x0000FF),
    YELLOW(0xFFFF00);

    fun containsRed() = (this.rgb and 0xFF0000 != 0)  // 3
}

fun main() {
    val red = Color.RED
    println(red)                                      // 4
    println(red.containsRed())                        // 5
    println(Color.BLUE.containsRed())                 // 6
    println(Color.YELLOW.containsRed())               // 7
}

1. Defines an enum class with a property and a method.
2. Each enum constant must pass an argument for the constructor parameter.
3. Enum class members are separated from the constant definitions by a semicolon.
4. The default toString returns the name of the constant, here "RED".
5. Calls a method on an enum constant.
6. Calls a method via enum class name.
7. The RGB values of RED and YELLOW share first bits (FF) so this prints 'true'.