Steps

1. Create a trait which will define the methods for a data access layer

Let's start with defining a simple trait called DonutShoppingCartDao and it will provide the method signatures to represent a Data Access Layer.

println("Step 1: Create a trait with type which will define the methods for a data access layer")
trait DonutShoppingCartDao[A] {

 def add(donut: A): Long

 def update(donut: A): Boolean

 def search(donut: A): A

 def delete(donut: A): Boolean

}

NOTE:

• The trait is defined with type parameter DonutShoppingCartDao[A]. If you are unsure about type parameters, please review the previous tutorial.

2. Create a second trait which will define the methods for checking donut inventory

Next, we create another trait called DonutInventoryService[A] which encapsulates methods to query donut inventories such as checkStockQuantity() method.

println("\nStep 2: Create a second trait which will define the methods for checking donut inventory")
trait DonutInventoryService[A] {

 def checkStockQuantity(donut: A): Int

}

NOTE:

• Trait DonutInventoryService[A] is also defined with type parameter.

3. Create a DonutShoppingCart class which extends multiple traits namely trait DonutShoppingCartDao and trait DonutInventoryService

We then define a class named DonutShoppingCart[A] which extends both the traits DonutShoppingCartDao[A]  and DonutInventoryService[A] as shown below.

println("\nStep 3: Create a DonutShoppingCart class which extends multiple traits namely trait DonutShoppingCartDao and trait DonutInventoryService")
class DonutShoppingCart[A] extends DonutShoppingCartDao[A] with DonutInventoryService[A] {

 override def add(donut: A): Long = {
  println(s"DonutShoppingCart-> add method -> donut: $donut")
  1
 }

 override def update(donut: A): Boolean = {
  println(s"DonutShoppingCart-> update method -> donut: $donut")
  true
 }

 override def search(donut: A): A = {
  println(s"DonutShoppingCart-> search method -> donut: $donut")
  donut
 }

 override def delete(donut: A): Boolean = {
  println(s"DonutShoppingCart-> delete method -> donut: $donut")
  true
 }

 override def checkStockQuantity(donut: A): Int = {
  println(s"DonutShoppingCart-> checkStockQuantity method -> donut: $donut")
  10
 }
}

NOTE:

• We've made use of the with keyword to extend multiple traits.

4. Create an instance of DonutShoppingCart and call the add, update, search and delete methods

You can now create an instance of DonutShoppingCart and call the add, update, search and delete methods which were inherited from trait DonutShoppingCartDao[A]

println("\nStep 4: Create an instance of DonutShoppingCart and call the add, update, search and delete methods")
val donutShoppingCart: DonutShoppingCart[String] = new DonutShoppingCart[String]()
donutShoppingCart.add("Vanilla Donut")
donutShoppingCart.update("Vanilla Donut")
donutShoppingCart.search("Vanilla Donut")
donutShoppingCart.delete("Vanilla Donut")

NOTE:

• To keep the example simple, we are passing a type of String although in a real world application you would most likely have custom type classes.

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

Step 4: Create an instance of DonutShoppingCart and call the add, update, search and delete methods
DonutShoppingCart-> add method -> donut: Vanilla Donut
DonutShoppingCart-> update method -> donut: Vanilla Donut
DonutShoppingCart-> search method -> donut: Vanilla Donut
DonutShoppingCart-> delete method -> donut: Vanilla Donut

5. Call the checkStockQuantity method which was inherited from trait DonutInventoryService

Since class DonutShoppingCart also extended the second trait DonutInventoryService[A], you also have access to the checkStockQuantity() method as shown below.

println("\nStep 5: Call the checkStockQuantity method which was inherited from trait DonutInventoryService")
donutShoppingCart.checkStockQuantity("Vanilla Donut")

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

Step 5: Call the checkStockQuantity method which was inherited from trait DonutInventoryService
DonutShoppingCart-> checkStockQuantity method -> donut: Vanilla Donut