Advanced Kotlin Class Features

Open your internet browser and search for Kotlin Playground. You’ll see different websites. Select the one provided by the Android Developers page. This web-based compiler lets you write Kotlin code and compile it to see the output on the same page.

Open the starter folder from the course material. You’ll see a Kotlin file with some code to get you started with this lesson. Here’s the code:

class FruitBox<A, B, C, D>(
  val firstItem: A,
  val secondItem: B,
  val numOfItems: C,
  val totalCost: D
) {
  fun printContents() {
    println("FruitBox contents: First Item = $firstItem, Second Item = $secondItem")
    println("Total Items = $numOfItems, Total Cost = $$totalCost")
  }
}

fun main() {
  val appleBananaBox = FruitBox("Apple", "Banana", 5, 10.5)
  appleBananaBox.printContents()
}

Now, you’ll create a companion object block within the FruitBox class with the following members:

companion object {
  const val BOX_CAPACITY = 10
  fun getDefaultBox(): FruitBox<String, String, Int, Double> {
    return FruitBox("Apple", "Banana", 8, 14.2)
  }
}

Here’s a code breakdown:

1. const val BOX_CAPACITY = 10 defines a constant property, BOX_CAPACITY, representing the maximum capacity of the fruit box.
2. fun getDefaultBox() is a function that returns a default FruitBox instance initialized with predefined values: "Apple", "Banana", 8, and 14.2.

Next, you need to update the main() function to display the results of the companion object members in the console. Update the main() function with the following code:

println("\nCalling Companion Object")
println("Box Capacity: ${FruitBox.BOX_CAPACITY}")

val defaultBox = FruitBox.getDefaultBox()
defaultBox.printContents()

Here’s a code breakdown:

1. println("\nCalling Companion Object"): Prints a message indicating that the companion object is being called.
2. println("Box Capacity: ${FruitBox.BOX_CAPACITY}") prints the box capacity by accessing the constant property BOX_CAPACITY from the FruitBox class using its companion object.
3. val defaultBox = FruitBox.getDefaultBox() calls the getDefaultBox() function from the companion object of FruitBox to create a default box instance.
4. defaultBox.printContents() calls the printContents() function on the defaultBox instance to print its contents.

Run the code to see the output to the console.