Android Development with Kotlin

Variance

Subtyping is a popular concept in the OOP paradigm. We define inheritance between two classes by extending the class:

    open class Animal(val name: String)    class Dog(name: String): Animal(name)

The Dog class extends the Animal class, so the Dog type is a subtype of Animal. This means that we can use an expression of type Dog whenever an expression of type Animal is required; for example, we can use it as a function argument or assign a variable of type Dog to a variable of type Animal:

    fun present(animal: Animal) {        println( "This is ${ animal. name } " )    }    present(Dog( "Pluto" )) // Prints: This is Pluto

Before we move on, we need to discuss the difference between class and type. Type is a more general term--it can be defined by ...