Steps

1. How to define a simple class to represent a Donut object

If you have followed the two previous tutorials on Companion Objects as listed above, you should be familiar with the Donut class below which has a print() method.

println("Step 1: How to define a simple class to represent a Donut object")
class Donut(name: String, productCode: Option[Long] = None){

def print = println(s"Donut name = $name, productCode = ${productCode.getOrElse(0)}, uuid = ${Donut.uuid}")

}

NOTE:

• But within the print statement above, we also print a unique ID for each donut as represented by the uuid field: uuid = ${Donut.uuid}
• But where does the uuid field come from? Let's proceed to Step 2 below which shows the Donut Companion Object.

2. How to declare fields and values in Companion Object

Notice that within the Donut Companion Object, we've defined a value named uuid but also marked it as private.

If you have used another programming language such as Java or .NET in the past, it should be of no surprise that the private keyword would hide the visibility of the field uuid.

But, given that a Companion Object works along side the class to which the Companion Object refers to, even though the uuid field is not visible to the outside world, it is still accessible within the Donut class as shown in Step 2.

println("\nStep 2: How to declare fields and values in companion object")
object Donut {

 private val uuid = 1

 def apply(name: String, productCode: Option[Long]): Donut = {
  new Donut(name, productCode)
 }

 def apply(name: String): Donut = {
  new Donut(name)
 }
}

3. How to create instances of the Donut class using the Companion Object

Next let's create a couple of instances of Donut class using the Donut's Companion Object apply() method similar to what you've learned from the tutorial on Learn How To Create And Use Companion Objects.

println("\nStep 3: How to create instances of the Donut class using the companion object")
val glazedDonut = Donut("Glazed Donut", Some(1111))
val vanillaDonut = Donut("Vanilla Donut")

4. How to call function on each Donut object

When you call the print() method on each of the Donut instances, you will notice that the uuid value is also being printed in the console window.

println("\nStep 4: How to call function on each Donut object")
glazedDonut.print
vanillaDonut.print

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

Step 4: How to call function on each Donut object
Donut name = Glazed Donut, productCode = 1111, uuid = 1
Donut name = Vanilla Donut, productCode = 0, uuid = 1