Ask for inheritance

less than 1 minute read

Inheritance is a mechanism for creating a new class by reusing and modifying an existing class.

In order to do this, in Kotlin, the base class must be open. A non-open class doesn’t allow inheritance – it is closed by default. This differs from most other object-oriented languages.

For example, in Java, a class is inheritable automatically unless you explicitly forbid by declaring it as final.

open class Solider {
  val id: String = 1
  val age: Int = 25
}

class Corporal: Solider()
class Sergeant: Solider()

Inheritance gets interesting when you start overriding functions, which means redefining a function from a base class to do something different a derived class.

open class Solider {
  protected var energy = 0
  open fun reportTo() = "General"
  fun energyLevel() = "Energy: $energy"
}

class Corporal: Solider() {
  override fun reportTo() = "Sergeant"
  fun serviceYear() = 10
}

Summary

Kotlin imposes an additional constraint when overriding functions. Just like base class, you cannot override function from a base class unless it is defined as open. Inheritance and overriding cannot be accomplished in Kotlin without clear intentions.

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