In Kotlin, classes are final by default which means they cannot be inherited from.

To allow inheritance on a class, use the open keyword.

open class Thing {
    // I can now be extended!
}

Note: abstract classes, sealed classes and interfaces will be open by default.

Defining the base class:

open class BaseClass {
    val x = 10
}

Defining the derived class:

class DerivedClass: BaseClass() {
    fun foo() {
        println("x is equal to " + x)
    }
}

Using the subclass:

fun main(args: Array<String>) {
    val derivedClass = DerivedClass()
    derivedClass.foo() // prints: 'x is equal to 10'
}

Defining the base class:

open class Person {
    fun jump() {
        println("Jumping...")
    }
}

Defining the derived class:

class Ninja: Person() {
    fun sneak() {
        println("Sneaking around...")
    }
}

The Ninja has access to all of the methods in Person

fun main(args: Array<String>) {
    val ninja = Ninja()
    ninja.jump()  // prints: 'Jumping...'
    ninja.sneak() // prints: 'Sneaking around...'
}

Overriding properties (both read-only and mutable):

abstract class Car {
    abstract val name: String;
    open var speed: Int = 0;
}

class BrokenCar(override val name: String) : Car() {
    override var speed: Int
        get() = 0
        set(value) {
            throw UnsupportedOperationException("The car is bloken")
        }
}

fun main(args: Array<String>) {
    val car: Car = BrokenCar("Lada")
    car.speed = 10
}

Overriding methods:

interface Ship {
    fun sail()
    fun sink()
}

object Titanic : Ship {

    var canSail = true

    override fun sail() {
        sink()
    }

    override fun sink() {
        canSail = false
    }
}