Kotlin Enum Classes | Tutorial

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Kotlin Generics

Kotlin Object Expressions and Object Declarations

Kotlin Enum Classes

The most basic use of an enum class is to implement a type-safe enum.

Enum constants are separated by commas, and each enum constant is an object.

enum class Color{ RED,BLACK,BLUE,GREEN,WHITE }

Enum Initialization

Each enum is an instance of the enum class and can be initialized:

enum class Color(val rgb: Int) {RED(0xFF0000),GREEN(0x00FF00),BLUE(0x0000FF)}

The default name is the enum constant name, and the value starts from 0. If you need to specify a value, you can use its constructor:

enum class Shape(value:Int){ ovel(100), rectangle(200)}

Enums also support declaring their own anonymous classes and corresponding methods, as well as overriding base class methods. For example:

enum class ProtocolState { WAITING { override fun signal() = TALKING }, TALKING { override fun signal() = WAITING }; abstract fun signal(): ProtocolState}

If an enum class defines any members, a semicolon must be used to separate the enum constant definitions from the member definitions.

Using Enum Constants

Enum classes in Kotlin have synthetic methods that allow iterating over the defined enum constants and obtaining enum constants by their name.

EnumClass.valueOf(value: String): EnumClass // Converts the specified name to an enum value. Throws IllegalArgumentException if no match is found.
EnumClass.values(): Array<EnumClass> // Returns the enum values in an array.

Obtaining enum-related information:

val name: String // Gets the enum name.
val ordinal: Int // Gets the ordinal of the enum value in the array of all enum constants.

Example

enum class Color{ RED,BLACK,BLUE,GREEN,WHITE }

fun main(args: Array<String>) {
    var color:Color=Color.BLUE
    println(Color.values())
    println(Color.valueOf("RED"))
    println(color.name)
    println(color.ordinal)
}

Since Kotlin 1.1, you can use the enumValues<T>() and enumValueOf<T>() functions to access constants in an enum class in a generic way:

enum class RGB { RED, GREEN, BLUE }

inline fun <reified T : Enum<T>> printAllValues() {
    print(enumValues<T>().joinToString { it.name })
}

fun main(args: Array<String>) {
    printAllValues<RGB>() // Output: RED, GREEN, BLUE
}

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