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class Stack[A] {
  private var elements: List[A] = Nil
  def push(x: A) { elements = x :: elements }
  def peek: A = elements.head
  def pop(): A = {
    val currentTop = peek
    elements = elements.tail
    currentTop
  }
}

val stack = new Stack[Int]
stack.push(1)
stack.push(2)
println(stack.pop)  // prints 2
println(stack.pop)  // prints 1

class Fruit
class Apple extends Fruit
class Banana extends Fruit

val stack2 = new Stack[Fruit]
val apple = new Apple
val banana = new Banana

stack2.push(apple)
stack2.push(banana)
println(stack2.pop.getClass.getSimpleName)  // prints Banana
println(stack2.pop.getClass.getSimpleName)  // prints Apple

//class Foo[+A] // A covariant class
//class Bar[-A] // A contravariant class
//class Baz[A]  // An invariant class


println("# Covariance")
abstract class Animal {
  def name: String
}
case class Cat(name: String) extends Animal
case class Dog(name: String) extends Animal

def printAnimalNames(animals: List[Animal]): Unit = {
  println("##" + animals )
  animals.foreach { animal =>
    println(animal.name)
  }
}

val cats: List[Cat] = List(Cat("Whiskers"), Cat("Tom"))
val dogs: List[Dog] = List(Dog("Fido"), Dog("Rex"))

printAnimalNames(cats)
printAnimalNames(dogs)


println("# Contravariance")

abstract class Printer[-A] {
  def print(value: A): Unit
}

class AnimalPrinter extends Printer[Animal] {
  def print(animal: Animal): Unit =
    println("The animal's name is: " + animal.name)
}

class CatPrinter extends Printer[Cat] {
  def print(cat: Cat): Unit =
    println("The cat's name is: " + cat.name)
}

val myCat: Cat = Cat("Boots")

def printMyCat(printer: Printer[Cat]): Unit = {
  printer.print(myCat)
}

val catPrinter: Printer[Cat] = new CatPrinter
val animalPrinter: Printer[Animal] = new AnimalPrinter

printMyCat(catPrinter)
printMyCat(animalPrinter)

println("# Invariance")

abstract class SmallAnimal extends Animal
case class Mouse(name: String) extends SmallAnimal
var smallAni:SmallAnimal = new Mouse("miky")

println(smallAni.name)



//val cat: Cat = catContainer.getValue // Oops, we'd end up with a Dog assigned to a Cat