Covariance vs Contravariance

Variance.kt

package com.tomek

abstract class Animal(val size: Int)

class Dog(val cuteness: Int): Animal(100)

class Spider(val terrorFactor: Int): Animal(1)


// Covariance

val dogList: List<Dog> = listOf()
val animalList: List<Animal> = dogList // Works!

val spiderList: List<Spider> = animalList // Compiler error, cannot assign List<Animal> to List<Spider> 


// Contravariance

interface Compare<in T> {
    fun compare(first: T, second: T): Int
}

val dogCompare: Compare<Dog> = object: Compare<Dog> {
    override fun compare(first: Dog, second: Dog): Int {
        return first.cuteness - second.cuteness
    }
}

val animalCompare: Compare<Animal> = dogCompare // Compiler error, cannot assign List<Dog> to List<Animal> 

val spider: Compare<Spider> = object: Compare<Animal> { // Works!
    override fun compare(first: Animal, second: Animal): Int {
        return first.size - second.size
    }
}