Nested class

Nested Class

A nested class enables us to group more logically, hence it makes it more readable and refined structure. A nested class is simply a class within the namespace of outer class.

class Outer(private val code: String) {
  class Inner {
    fun hello() = "hello"
  }
}

Local Class

A local class is the class that is nested inside function. Within function, one thing to take note is that we couldn’t have interface. The local classes are not visible outside the function, so we cannot return them.

fun thisIsLocal() {
  open class Animal
  class Dog : Animal()

  val aDog: Animal = Dog()
}

Class inside Interface

Class can be nested inside interface.

interface Item {
  val type = Type
  data class Type(val type: String)
}

class Hammer(material: String) : Item {
  override val type = Item.Type(material)
}

Nested Enum

enum are class, so they can be nested inside other class.

class FootballClub(
  val name: String,
  val size: Size = Small
) {
  enum class Size {
    Large,
    Medium,
    Small
  }

  fun grow() : FootballClub {
    val newSize = values()[
      (size.ordinal - 1).coerceAtLeast(Large.ordinal)
    ]

    return FootballClub(name, newSize)
  }
}