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 its enum class, and they can be initialized:

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

By default, the 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 with 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 the enum class defines any members, you need to separate the enum constant definitions from the member definitions with a semicolon.

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Using Enum Constants

Enum classes in Kotlin have synthetic methods that allow you to iterate over the defined enum constants and obtain an enum constant by its name.

EnumClass.valueOf(value: String): EnumClass  // 转换指定 name 为枚举值，若未匹配成功，会抛出IllegalArgumentException
EnumClass.values(): Array<EnumClass>        // 以数组的形式，返回枚举值

Getting enum-related information:

val name: String //获取枚举名称
val ordinal: Int //获取枚举值在所有枚举数组中定义的顺序

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 useenumValues<T>()andenumValueOf<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>() // 输出 RED, GREEN, BLUE
}

Other Extensions