Kotlin generics, inheritence and interfaces example showing multiple interface implementation, inheritance and co-/contra-variance in generics

GenericsExample.kt

interface MustImplementAdd {
    fun theAddLol() : Int
}

interface MustImplementDivide {
    fun divide(a: Float, b: Float) : Float
}

abstract class Calculator(val name: String) {
    abstract fun calculate() : Double
    fun getMyName(): String = "my name is $name"
}

class BoxIn<in S : MustImplementAdd> {
    fun x(m : S) {
        m.theAddLol()
    }
}

class BoxOut<out S : MustImplementAdd>(val s : S)

class BoxInAndOut<in T : MustImplementDivide, out S : MustImplementAdd>(val s : S) {
    fun x(t: T) {
        t.divide(1.2f, 2.0f)
    }
}

class AdditionAndDivisionCalculator(val a: Int, val b: Int, nameOfCalculator: String) : MustImplementAdd, MustImplementDivide, Calculator(nameOfCalculator) {
    override fun divide(a: Float, b: Float): Float {
        return a / b
    }

    override fun calculate(): Double {
        return a + b * 2.5
    }

    override fun theAddLol(): Int {
        return a + b
    }
}

class DivisionCalculator() : MustImplementDivide {
    override fun divide(a: Float, b: Float): Float {
        return a / b
    }
}

class AdditionCalculator() : MustImplementAdd {
    override fun theAddLol(): Int {
        return 99
    }
}

val additionCalculator = AdditionAndDivisionCalculator(1, 2, "Dr. Misty")

val a = BoxIn<AdditionAndDivisionCalculator>() // BoxIn consumes some S that implements MustImplementAdd
val b = a.x(additionCalculator) // we give into BoxIn some S that implements MustImplementAdd

val c = BoxOut(additionCalculator).s // BoxOut produces some S that implements MustImplementAdd (Type of c is inferred to BoxOut<AdditionAndDivisionCalculator>)
val d = c.getMyName() // we can get out some S that implements MustImplementAdd

val e = BoxInAndOut<DivisionCalculator, AdditionCalculator>(AdditionCalculator())
val g = e.x(DivisionCalculator()) // put in an S, that implements MustImplementDivide
val f = e.s // gets out an S, that implements MustImplementAdd