Enum

Enum in Kotlin is a way to define a set of constant values. Enums are very useful, but not limited, as configuration values:

enum class Flour {
  WHEAT, CORN, CASSAVA
}

Each element is an object that extends the Flour class.

Like any object, they can extend interfaces:

interface Exotic {
  fun isExotic(): Boolean
}

enum class Flour : Exotic {
  WHEAT {
    override fun isExotic(): Boolean {
      return false 
    }
  },

  CORN {
    override fun isExotic(): Boolean {
      return false
    }
  },

  CASSAVA {
    override fun isExotic(): Boolean {
      return true
    }
  }
}

Enum can also have abstract methods:

enum class Flour: Exotic {
  WHEAT {
    override fun isGlutenFree(): Boolean {
      return false
    }

    override fun isExotic(): Boolean {
      return false
    }
  },

  CORN {
    override fun isGlutenFree(): Boolean {
      return true
    }

    override fun isExotic(): Boolean {
      return false
    }
  },

  CASSAVA {
    override fun isGlutenFree(): Boolean {
      return true
    }

    override fun isExotic(): Boolean {
      return true
    }
  };
 
  abstract fun isGlutenFree(): Boolean
}

Any method definition must be declared after the (;) separating the last element.

When enums are used with when expressions, Kotlin's compiler checks that all cases are covered (individually or with an else):

fun flourDescription(flour: Flour): String {
  return when(flour) { // error
    Flour.CASSAVA -> "A very exotic flavour"
  }
}

In this case, we're only checking for CASSAVA and not the other elements; therefore, it fails:

fun flourDescription(flour: Flour): String {
  return when(flour) {
    Flour.CASSAVA -> "A very exotic flavour"
    else -> "Boring"
  }
}