Android Development with Kotlin

Sealed classes

A sealed class is a class with a limited number of subclasses (sealed subtyping hierarchy). Prior to Kotlin 1.1, these subclasses had to be defined inside a sealed class body. Kotlin 1.1 weakened this restriction and allowed us to define sealed class subclasses in the same file as a sealed class declaration. All the classes are declared close to each other, so we can easily see all possible subclasses by simply looking at one file:

    //vehicle.kt 
    sealed class Vehicle()
    class Car : Vehicle()
    class Truck : Vehicle()
    class Bus : Vehicle()

To mark a class as sealed, simply add a sealed modifier to the class declaration header. The preceding declaration means that the Vehicle class can be only extended by three classes, Car, Truck ...