Problem Statement

An object is polymorphic when its specific type may vary. The types a specific value may take, is called class. It is trivial to copy an object if its type is known: Here x is not polymorphic, so y is declared of same type (int) as x. But if the specific type of x were unknown, then y could not be declared of any specific type. The task: let a polymorphic object contain an instance of some specific type S derived from a type T. The type T is known. The type S is possibly unknown until run time. The objective is to create an exact copy of such polymorphic object (not to create a reference, nor a pointer to). Let further the type T have a method overridden by S. This method is to be called on the copy to demonstrate that the specific type of the copy is indeed S.

Let's start with the solution:

object PolymorphicCopy {

  def main(args: Array[String]) {
    val a: Animal = Dog("Rover", 3, "Terrier")
    val b: Animal = a.copy() // calls Dog.copy() because runtime type of 'a' is Dog
    println(s"Dog 'a' = $a") // implicitly calls Dog.toString()
    println(s"Dog 'b' = $b") // ditto
    println(s"Dog 'a' is ${if (a == b) "" else "not"} the same object as Dog 'b'")
  }

  case class Animal(name: String, age: Int) {

    override def toString = s"Name: $name, Age: $age"
  }

  case class Dog(override val name: String, override val age: Int, breed: String) extends Animal(name, age) {

    override def toString = super.toString() + s", Breed: $breed"
  }

}