Nullable Types and Casts

When it comes to casts and nullable types, you can use the nullable form of as (as?):

  val nullableList : MutableList<String>? = mutableListOf()

  nullableList?.add("this is not null")
  nullableList?.add("this is also not null")

  val simplerList = nullableList as? List<String>

  println(simplerList)

In this case, simplerList winds up being a List? of String objects. If the object being cast is null, you wind up with a null value.

as? is also useful for cases where the object might not be of the desired type:

open class Animal

class Frog : Animal()

class Axolotl : Animal()

fun main() {
  val kermit : Animal = Frog()

  val notAnAxolotl = kermit as? Axolotl

  println(notAnAxolotl)
}

as? results in a null value if either:

• The object being cast itself is null, or
• The object being cast is not of the type that you are trying to cast it to

In this case, kermit is not an Axolotl. As a result, notAnAxolotl winds up being null, with the variable being typed as Axolotl?.