Steps

1. How to create a wrapper String class which will extend the String type

Let us create a simple wrapper class called DonutString which will take the String type as its parameter and then provide an isFavoriteDonut() function.

println("Step 1: How to create a wrapper String class which will extend the String type")
class DonutString(s: String) {

  def isFavoriteDonut: Boolean = s == "Glazed Donut"

}

NOTE:

• If you are coming from a Java or .NET programming background, you would perhaps be pleasantly surprised that in Scala you can simply compare two Strings using == as opposed to using equals() method.

2. How to create an implicit function to convert a String to the wrapper String class

It is a good practice to encapsulate your implicit functions or conversions into a singleton using object. You can also make use of package object which we will see in upcoming tutorials.

As such, let us define an implicit function named stringToDonutString which will take the String type as its parameter and wire it through a new instance of the wrapper String class named DonutString from Step 1.

println("\nStep 2: How to create an implicit function to convert a String to the wrapper String class")
object DonutConverstions {
  implicit def stringToDonutString(s: String) = new DonutString(s)
}

NOTE:

• Defining an implicit function is similar to defining any other functions except that we've prefixed the function signature using the implicit keyword.

3. How to import the String conversion so that it is in scope

In order to use the implicit String function which will convert a String type into a DonutString type, you will have to have the implicit function from Step 2 in scope. This can be achieved using the import keyword as shown below:

println("\nStep 3: How to import the String conversion so that it is in scope")
import DonutConverstions._

NOTE:

• As part of the import expression, we are using the wildcard operator _ which will import any values or implicit functions.

4. How to create String values

Let us create two immutable values of type String, one for Glazed Donut and the other for Vanilla Donut.

println("\nStep 4: How to create String values")
val glazedDonut = "Glazed Donut"
val vanillaDonut = "Vanilla Donut"

NOTE:

• By now, you should be familiar with declaring variables and immutable values.
• Feel free to review the tutorial on declaring variables.

5. How to access the custom String function called isFavaoriteDonut

How would you access the isFavoriteDonut() function from Step 1? Simple - just call it on the Glazed Donut or Vanilla Donut String values.

println("\nStep 5: How to access the custom String function called isFavaoriteDonut")
println(s"Is Glazed Donut my favorite Donut = ${glazedDonut.isFavoriteDonut}")
println(s"Is Vanilla Donut my favorite Donut = ${vanillaDonut.isFavoriteDonut}")

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

Step 5: How to access the custom String function called isFavaoriteDonut
Is Glazed Donut my favorite Donut = true
Is Vanilla Donut my favorite Donut = false

NOTE:

• The custom isFavoriteDonut() function looks built-into the String class.
• However, we did not have to manually modify the source code of the String class.
• Instead, we've used the secret powers of Scala's implicit function to extend the String class.