Inheritance (Extending a Class)

In this lesson you will learn how to extend a class through inheritance in Kotlin, so classes that share characteristics can reuse code. This is for anyone searching for "Kotlin inheritance how to use".

You inherit with : ParentClassName. Marking the parent class and its methods open is what allows a child class to override them. Gathering the shared parts into a parent class saves you writing the same code repeatedly.

The sample code makes open class Animal the parent, has class Dog(name: String) : Animal(name) inherit from it, and replaces the parent's method with override fun speak(). The key point is that you keep the parent's functionality and change only the parts you need.

A common stumbling block for beginners is forgetting the open keyword. Kotlin classes are final by default and cannot be inherited, so allowing inheritance requires stating open explicitly.

In real-world development this design, which prevents unintended inheritance, makes for safer class design.

📖 Reference code

open class Animal(val name: String) {
    open fun speak(): String = "${name} makes a sound"
}

class Dog(name: String) : Animal(name) {
    override fun speak(): String = "${name} says \"Woof\""
}

fun main() {
    val dog = Dog("Rex")
    println(dog.speak())
}

✍️ Your code

Type your code, then press "Run"

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