Inheritance, overrides and safe casts

A school has a general person description and a specialized teacher description. Class inheritance can express a specialization, and an override replaces an inherited implementation. It should model a genuine substitutable relationship rather than merely avoid copying a few lines.

Kotlin classes and members are final by default. Mark a class and overridable member open when specialization is intended, and use override in the subclass. A safe cast with as? returns null on a mismatch, while an unchecked as can fail. Is checks type membership and can enable a smart cast.

Save the complete source as Main.kt and use the Kotlin/JVM environment. No separate input file is needed.

Kotlin/JVM · complete program

open class Person {
  open fun role(): String = "Person"
}
class Teacher : Person() {
  override fun role(): String = "Teacher"
}
fun main() {
  val person: Person = Teacher()
  println(person.role())
  println(person is Teacher)
  println((person as? Teacher)?.role() ?: "Other")
}

Expected output

Teacher
true
Teacher

Follow the execution

The reference’s declared type is Person, but its instance is Teacher. The override supplies Teacher at runtime, the is check succeeds and the safe cast allows the same role call.

Try this in the Kotlin compiler by pasting the complete program.

Your task: What does as? Teacher produce for an ordinary Person instance?

Kotlin/JVM · complete program

open class Person {
  open fun role(): String = "Person"
}
class Teacher : Person() {
  override fun role(): String = "Teacher"
}
fun main() {
  val person: Person = Teacher()
  println(person.role())
  println(person is Teacher)
  println((person as? Teacher)?.role() ?: "Other")
}

Worked answer

It returns null. A safe cast checks the existing instance rather than creating a teacher.

Inheritance should express a substitutable relationship, not merely reuse a few lines. Prefer an interface or composition for unrelated capabilities. Do not assume Java’s or Swift’s default overridability rules apply.

It returns null. A safe cast checks the existing instance rather than creating a teacher. Inheritance should express a substitutable relationship, not merely reuse a few lines.

Further reading: official Kotlin documentation for this topic.