Properties

As discussed previously, classes can have a state. In Kotlin, a class's state is represented by properties. Let's have a look at the blueberry cupcake example:

class BlueberryCupcake {
  var flavour = "Blueberry"
}

The  BlueberryCupcake class has an has-a property flavour of type String.

Of course, we can have instances of the BlueberryCupcake class:

fun main(args: Array<String>) {
    val myCupcake = BlueberryCupcake()
    println("My cupcake has ${myCupcake.flavour}")
}

Now, because we declare the flavour property as a variable, its internal value can be changed at runtime:

fun main(args: Array<String>) {
    val myCupcake = BlueberryCupcake()
    myCupcake.flavour = "Almond"
    println("My cupcake has ${myCupcake.flavour}")
}

That is impossible in real life. Cupcakes do not change their flavor (unless they become stale). If we change the flavour property to a value, it cannot be modified:

class BlueberryCupcake {
    val flavour = "Blueberry"
}

fun main(args: Array<String>) {
    val myCupcake = BlueberryCupcake()
    myCupcake.flavour = "Almond" //Compilation error: Val cannot be reassigned
    println("My cupcake has ${myCupcake.flavour}")
}

Let's declare a new class for almond cupcakes:

class AlmondCupcake {
    val flavour = "Almond"
}

fun main(args: Array<String>) {
    val mySecondCupcake = AlmondCupcake()
    println("My second cupcake has ${mySecondCupcake.flavour} flavour")
}

There is something fishy here. BlueberryCupcake and AlmondCupcake are identical in structure; only an internal value is changed.

In real life, you don't have different baking tins for different cupcake flavors. The same good quality baking tin can be used for various flavors. In the same way, a well-designed Cupcake class can be used for different instances:

class Cupcake(flavour: String) { 
  val flavour = flavour
}

The Cupcake class has a constructor with a parameter, flavour, that is assigned to a flavour value.

Because this is a very common idiom, Kotlin has a little syntactic sugar to define it more succinctly:

class Cupcake(val flavour: String)

Now, we can define several instances with different flavors:

fun main(args: Array<String>) {
    val myBlueberryCupcake = Cupcake("Blueberry")
    val myAlmondCupcake = Cupcake("Almond")
    val myCheeseCupcake = Cupcake("Cheese")
    val myCaramelCupcake = Cupcake("Caramel")
}