Covar.scala

/**

  Here is an object that has a method mixFoods which will accept
  a subclass of a GardenItem and return a list of either
  GardenItems or subclasses of GardenItems.

  */
object CovarianceExample {
  /**
    Covariance example (compiles)

    def mixFoods[T >: GardenItem](food: T): List[T] = {

    This method definition states that within the context of the
    mixFoods method, there is a type T that is a superclass of a GardenItem.

    We then go on to say that this method accepts an argument which
    is of type T, a subclass of GardenItem and it returns a List of
    either GardenItems or subclasses of GardenItems.

    */
  def mixFoods[T >: GardenItem](food: T): List[T] = {
    food match {
      case x: FruitSalad => {
        /**
          Watermelon is a subclass of GardenItem so this is valid
          to the compiler.
          */
        List(new Watermelon, food)
      }
      case x: Cucumber => {
        /**
          Cocktail is a subclass of Cucumber, which is a subclass of
          GardenItem so this is also valid to the compiler.
          */
        List(food, new Cocktail)
      }
      case _ => List()
    }
  }
}