Kotlin Programming: The Big Nerd Ranch Guide, First Edition

in and out

To further customize your generic type parameters, Kotlin provides the keywords in and out. To see how they work, create a simple generic Barrel class in a new file called Variance.kt:

Listing 17.15  Defining Barrel (Variance.kt)

class Barrel<T>(var item: T)

To experiment with Barrel, add a main function. In main, define a Barrel to hold a Fedora and another Barrel to hold Loot:

Listing 17.16  Defining Barrels in main (Variance.kt)

class Barrel<T>(var item: T)

fun main(args: Array<String>) {
    var fedoraBarrel: Barrel<Fedora> = Barrel(Fedora("a generic-looking fedora", 15))
    var lootBarrel: Barrel<Loot> = Barrel(Coin(15))
}

While a Barrel<Loot> can hold any kind of loot, the particular instance defined here ...