Steps

1. How to define an abstract class called Donut 

When dealing with class and object hierarchies, you typically have a base class which encapsulates common behaviour. As an example, let's create an abstract class name Donut which defines a method signature for printName.

println("Step 1: How to define an abstract class called Donut")
 abstract class Donut(name: String) {

   def printName: Unit

}

NOTE:

• Any class which extends the abstract class Donut will have to provide an implementation for the printName method.

2. How to extend abstract class Donut and define a case class called VanillaDonut

To create a sub-class of the abstract Donut class from Step 1, you have to use the extends keyword, pass-through the name constructor parameter and also provide an implementation for the printName method.

As shown in the example below, the case class VanillaDonut extends the abstract class Donut and also provides an implementation for the printName method.

println("\nStep 2: How to extend abstract class Donut and define a case class called VanillaDonut")
 case class VanillaDonut(name: String) extends Donut(name) {

   override def printName: Unit = println(name)

 }

NOTE:

• Unlike the previous tutorial on Class Inheritance, we did not have to create a Companion Object for the VanillaDonut case class because the case class will automatically provide a companion object.

3. How to extend abstract class Donut and define another case class of Donut called GlazedDonut

Similar to Step 2, let's create another case class named GlazedDonut which extends the abstract Donut class.

println("\nStep 3: How to extend abstract class Donut and define another case class called GlazedDonut")
 case class GlazedDonut(name: String) extends Donut(name) {

   override def printName: Unit = println(name)

 }

NOTE:

• Similar to Step 2, we did not have to create a Companion Object for the GlazedDonut class.

4. How to instantiate Donut objects

Let's create two Donut objects, one using the VanillaDonut class and the other one using the GlazedDonut class. Note that we are specifying the type for both vanillaDonut and glazedDonut to be of base type Donut.

println("\nStep 4: How to instantiate Donut objects")
val vanillaDonut: Donut = VanillaDonut("Vanilla Donut")
vanillaDonut.printName

val glazedDonut: Donut = GlazedDonut("Glazed Donut")
glazedDonut.printName

You should see the following output when you run your Scala application in IntelliJ:

Step 4: How to instantiate Donut objects
Vanilla Donut
Glazed Donut

NOTE:

• Since VanillaDonut and GlazedDonut are sub-classes of the base class Donut, they both have access to the printName method.

5. How to define a ShoppingCart type class which expects Donut types

The above steps should be familiar already if you have followed the previous tutorials on Class Inheritance and Case Class Inheritance.

What if you had to define a class which expects a particular type parameter? As an example, let us define a ShoppingCart class which expects a sequence of Donut types.

println("\nStep 5: How to define a ShoppingCart type class which expects Donut types")
class ShoppingCart[D <: Donut](donuts: Seq[D]) {

  def printCartItems: Unit = donuts.foreach(_.printName)

}

NOTE:

• With the notation [D <: Donut], we are restricting only Donut types to be passed-through to the ShoppingCart class.

6. How to create instances or objects of ShoppingCart class

The example below shows how to instantiate a ShoppingCart class of type Donut.

println("\nStep 6: How to create instances or objects of ShoppingCart class")
val shoppingCart: ShoppingCart[Donut] = new ShoppingCart(Seq[Donut](vanillaDonut, glazedDonut))
shoppingCart.printCartItems

You should see the following output when you run your Scala application in IntelliJ:

Step 6: How to create instances or objects of ShoppingCart class
Vanilla Donut
Glazed Donut

NOTE:

• Assume you want a ShoppingCart of VanillaDonut but would also like to inject Donut types.

val shoppingCart: ShoppingCart[VanillaDonut] = new ShoppingCart[Donut](Seq(glazedDonut))

• You will get a compiler error when trying to have a ShoppingCart[VanillaDonut] reference a ShoppingCart[Donut]

7. How to enable contra-variance on ShoppingCart

The example below shows how to instantiate a ShoppingCart2 class of type VanillaDonut by first enabling contra-variance for Donut types on ShoppingCart2 class.

println(s"\nStep 7: How to enable contra-variance on ShoppingCart")
class ShoppingCart2[-D <: Donut](donuts: Seq[D]) {

  def printCartItems: Unit = donuts.foreach(_.printName)

}
 
val shoppingCart2: ShoppingCart2[VanillaDonut] = new ShoppingCart2[Donut](Seq(glazedDonut))
shoppingCart2.printCartItems

You should see the following output when you run your Scala application in IntelliJ:

Step 7: How to enable contra-variance on ShoppingCart
Glazed Donut

NOTE:

• We've enabled contra-variance of type Donuts using the notation [-D <: Donut]
• In other words, you can now have a ShoppingCart of type VainllaDonut ShoppingCart[VanillaDonut] reference ShoppingCart of type Donut ShoppingCart2[Donut]