Enum classes

Enumeration is a specific type of class; a variable of a given enum type is limited to a set of predefined constants—those that have been defined by the type. To define an enumeration, you could use enum class keywords, as in the following example where we create a type for all the days in a week:

    enum class Day {  MONDAY, TUESDAY, WEDNESDAY, THURSDAY,  FRIDAY, SATURDAY, SUNDAY} 

Enumeration, like all classes, can take a constructor parameter. We can define an enum class to represent the planets in our solar system, and for each planet we retain the total mass and radius:

    public enum class Planet(val mass: Double, val radius: Double) { 
      MERCURY(3.303e+23, 2.4397e6), VENUS(4.869e+24, 6.0518e6),  EARTH(5.976e+24, 6.37814e6), MARS(6.421e+23, 3.3972e6),  JUPITER(1.9e+27, 7.1492e7), SATURN(5.688e+26, 6.0268e7),  URANUS(8.686e+25, 2.5559e7), NEPTUNE(1.024e+26, 2.4746e7); 
    } 

I made the two val parameters so they are exposed as properties. All enumeration instances come with two properties predefined. One is name of the String type, and the second one is ordinal of the int type. The former returns the name of the instance, and the latter gives you the position in the enumeration's type declaration.

Similar to Java, Kotlin provides you with helper methods to work with enumeration classes. To retrieve an enum value based on the name, you will need to use the following:

    Planet.valueOf("JUPITER") 

To get all the values defined, you will need to write the following:

    Planet.values()

Just like any class, enumeration types can inherit an interface and implement it anonymously for each enum value. Here is an example of how you could achieve this:

    interface Printable { 
      fun print(): Unit 
    } 
 
    public enum class Word : Printable { 
      HELLO { 
        override fun print() { 
          println("Word is HELLO") 
        } 
      }, 
      BYE { 
        override fun print() { 
          println("Word is BYE") 
        } 
      } 
    } 
 
    val w= Word.HELLO 
    w.print()