Scala Design Patterns - Second Edition

Subtype polymorphism

This is the polymorphism every developer knows about, and it's related to overriding methods in concrete class implementations. Consider the following simple hierarchy:

abstract class Item {  def pack: String}class Fruit extends Item {  override def pack: String = "I'm a fruit and I'm packed in a bag."}class Drink extends Item {  override def pack: String = "I'm a drink and I'm packed in a bottle."}

Now, let's have a shopping basket of items and call pack for each of them:

object SubtypePolymorphismExample {  def main(args: Array[String]): Unit = {    val shoppingBasket: List[Item] = List(      new Fruit,      new Drink    )    shoppingBasket.foreach(i => System.out.println(i.pack))  }}

As you can see, here we can use the abstract type and just ...