枚举类

枚举类的最基本应用是实现类型安全的多项目集合。

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

其中每一个常量（NORTH，SOUTH……）都是一个对象。每一个常量用逗号“,”分隔。

初始化

因为每一条枚举（RED，GREEN……）都是枚举类的实例，所以他们可以被初始化。

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

匿名类

枚举实例也可以被声明为他们自己的匿名类，并同时包含他们相应原本的方法和覆盖基本方法。

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

  TALKING {
    override fun signal() = WAITING
  };

  abstract fun signal(): ProtocolState
}

with their corresponding methods, as well as overriding base methods. Note that if the enum class defines any members, you need to separate the enum constant definitions from the member definitions with a semicolon, just like in Java.

Working with Enum Constants

Just like in Java, enum classes in Kotlin have synthetic methods allowing to list the defined enum constants and to get an enum constant by its name. The signatures of these methods are as follows (assuming the name of the enum class is EnumClass):

EnumClass.valueOf(value: String): EnumClass
EnumClass.values(): Array<EnumClass>

如果特定的对象名无法对应任何一个定义在枚举类中的枚举常量， valueOf() 方法会抛出一个异常 IllegalArgumentException。

每一个枚举常量在枚举类定义时都有一个属性去获得他们的名字和位置。

val name: String
val ordinal: Int

枚举常量也可以实现Comparable 接口。他们会依照在枚举类中的定义先后以自然顺序排列。