CoAndContravariant.kt

package main
//Covarient example start
interface Source<out T> {
    fun nextT(): T
}

class Complement<Int>(var num:Int):Source<Int>{
    override fun nextT()=num
    override fun toString()="${this::class.simpleName}"
}

fun demo(num: Source<Int>) {
    val objects: Source<Any> = num // This is OK, since T is an out-parameter)(Out means covarient)
    println(objects)
    println(objects.nextT())
}//covarient example end

//Contravariant
interface Comparable<in T> {// in means contravarient
    operator fun compareTo(other: T): Int
}

fun demo2(x: Comparable<Number>) {
    println(x.compareTo(1.0)) // 1.0 has type Double, which is a subtype of Number
    // Thus, you can assign x to a variable of type Comparable<Double>
    val y: Comparable<Double> = x // OK!
    println(y)
}

class Complement2<Number>(var num:Int):Comparable<Number>{
    override fun compareTo(other:Number)=0
}
//Contravariant end
fun main(){
  demo(Complement<Int>(123))
  demo2(Complement2<Number>(2))
}