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In this chapter, you'll learn about open classes, how they work with interfaces, and other special types of classes available in Kotlin.

Open classes

If you can't use interfaces or abstract classes, you can explicitly make a class inheritable by declaring it as open. To do this, use the open keyword before your class declaration:

open class Vehicle(val make: String, val model: String)

To create a class that inherits from another, add a colon after your class header followed by a call to the constructor of the parent class that you want to inherit from. In this example, the Car class inherits from the Vehicle class:

open class Vehicle(val make: String, val model: String)

class Car(make: String, model: String, val numberOfDoors: Int) : Vehicle(make, model)

fun main() {
    // Creates an instance of the Car class
    val car = Car("Toyota", "Corolla", 4)

    // Prints the details of the car
    println("Car Info: Make - ${car.make}, Model - ${car.model}, Number of doors - ${car.numberOfDoors}")
    // Car Info: Make - Toyota, Model - Corolla, Number of doors - 4
}

{kotlin-runnable="true" kotlin-min-compiler-version="1.3" id="kotlin-tour-classes-open-class"}

Just like when creating a normal class instance, if your class inherits from a parent class, then it must initialize all the parameters declared in the parent class header. So in the example, the car instance of the Car class initializes the parent class parameters: make and model.

Overriding inherited behavior

If you want to inherit from a class but change some of the behavior, you can override the inherited behavior.

By default, it's not possible to override a member function or property of a parent class. Just like with abstract classes, you need to add special keywords.

Member functions

To allow a function in the parent class to be overridden, use the open keyword before its declaration in the parent class:

open fun displayInfo() {}

{validate="false"}

To override an inherited member function, use the override keyword before the function declaration in the child class:

override fun displayInfo() {}

{validate="false"}

For example:

open class Vehicle(val make: String, val model: String) {
    open fun displayInfo() {
        println("Vehicle Info: Make - $make, Model - $model")
    }
}

class Car(make: String, model: String, val numberOfDoors: Int) : Vehicle(make, model) {
    override fun displayInfo() {
        println("Car Info: Make - $make, Model - $model, Number of Doors - $numberOfDoors")
    }
}

fun main() {
    val car1 = Car("Toyota", "Corolla", 4)
    val car2 = Car("Honda", "Civic", 2)

    // Uses the overridden displayInfo() function
    car1.displayInfo()
    // Car Info: Make - Toyota, Model - Corolla, Number of Doors - 4
    car2.displayInfo()
    // Car Info: Make - Honda, Model - Civic, Number of Doors - 2
}

{kotlin-runnable="true" id="kotlin-tour-class-override-function"}

This example:

  • Creates two instances of the Car class that inherit from the Vehicle class: car1 and car2.
  • Overrides the displayInfo() function in the Car class to also print the number of doors.
  • Calls the overridden displayInfo() function on car1 and car2 instances.

Properties

In Kotlin, it's not common practice to make a property inheritable by using the open keyword and overriding it later. Most of the time, you use an abstract class or an interface where properties are inheritable by default.

Properties inside open classes are accessible by their child class. In general, it's better to access them directly rather than override them with a new property.

For example, let's say that you have a property called transmissionType that you want to override later. The syntax for overriding properties is exactly the same as for overriding member functions. You can do this:

open class Vehicle(val make: String, val model: String) {
    open val transmissionType: String = "Manual"
}

class Car(make: String, model: String, val numberOfDoors: Int) : Vehicle(make, model) {
    override val transmissionType: String = "Automatic"
}

However, this is not good practice. Instead, you can add the property to the constructor of your inheritable class and declare its value when you create the Car child class:

open class Vehicle(val make: String, val model: String, val transmissionType: String = "Manual")

class Car(make: String, model: String, val numberOfDoors: Int) : Vehicle(make, model, "Automatic")

Accessing properties directly, instead of overriding them, leads to simpler and more readable code. By declaring properties once in the parent class and passing their values through the constructor, you eliminate the need for unnecessary overrides in child classes.

For more information about class inheritance and overriding class behavior, see Inheritance.

Open classes and interfaces

You can create a class that inherits a class and implements multiple interfaces. In this case, you must declare the parent class first, after the colon, before listing the interfaces:

// Define interfaces
interface EcoFriendly {
    val emissionLevel: String
}

interface ElectricVehicle {
    val batteryCapacity: Double
}

// Parent class
open class Vehicle(val make: String, val model: String)

// Child class
open class Car(make: String, model: String, val numberOfDoors: Int) : Vehicle(make, model)

// New class that inherits from Car and implements two interfaces
class ElectricCar(
    make: String,
    model: String,
    numberOfDoors: Int,
    val capacity: Double,
    val emission: String
) : Car(make, model, numberOfDoors), EcoFriendly, ElectricVehicle {
    override val batteryCapacity: Double = capacity
    override val emissionLevel: String = emission
}

Special classes

In addition to abstract, open, and data classes, Kotlin has special types of classes designed for various purposes, such as restricting specific behavior or reducing the performance impact of creating small objects.

Sealed classes

There may be times when you want to restrict inheritance. You can do this with sealed classes. Sealed classes are a special type of abstract class. Once you declare that a class is sealed, you can only create child classes from it within the same package. It's not possible to inherit from the sealed class outside of this scope.

A package is a collection of code with related classes and functions, typically within a directory. To learn more about packages in Kotlin, see Packages and imports.

{style="tip"}

To create a sealed class, use the sealed keyword:

sealed class Mammal

Sealed classes are particularly useful when combined with a when expression. By using a when expression, you can define the behavior for all possible child classes. For example:

sealed class Mammal(val name: String)

class Cat(val catName: String) : Mammal(catName)
class Human(val humanName: String, val job: String) : Mammal(humanName)

fun greetMammal(mammal: Mammal): String {
    when (mammal) {
        is Human -> return "Hello ${mammal.name}; You're working as a ${mammal.job}"
        is Cat -> return "Hello ${mammal.name}"   
    }
}

fun main() {
    println(greetMammal(Cat("Snowy")))
    // Hello Snowy
}

{kotlin-runnable="true" id="kotlin-tour-sealed-classes"}

In the example:

  • There is a sealed class called Mammal that has the name parameter in the constructor.
  • The Cat class inherits from the Mammal sealed class and uses the catName parameter in its own constructor as the name parameter from the Mammal class.
  • The Human class inherits from the Mammal sealed class and uses the humanName parameter in its own constructor as the name parameter from the Mammal class. It also has the job parameter in its constructor.
  • The greetMammal() function accepts an argument of Mammal type and returns a string.
  • Within the greetMammal() function body, there's a when expression that uses the is operator to check the type of mammal and decide which action to perform.
  • The main() function calls the greetMammal() function with an instance of the Cat class and name parameter called Snowy.

This tour discusses the is operator in more detail in the Null safety chapter.

{style ="tip"}

For more information about sealed classes and their recommended use cases, see Sealed classes and interfaces.

Enum classes

Enum classes are useful when you want to represent a finite set of distinct values in a class. An enum class contains enum constants, which are themselves instances of the enum class.

To create an enum class, use the enum keyword:

enum class State

Let's say that you want to create an enum class that contains the different states of a process. Each enum constant must be separated by a comma ,:

enum class State {
    IDLE, RUNNING, FINISHED
}

The State enum class has enum constants: IDLE, RUNNING, and FINISHED. To access an enum constant, use the class name followed by a . and the name of the enum constant:

val state = State.RUNNING

You can use this enum class with a when expression to define the action to take depending on the value of the enum constant:

enum class State {
    IDLE, RUNNING, FINISHED
}

fun main() {
    val state = State.RUNNING
    val message = when (state) {
        State.IDLE -> "It's idle"
        State.RUNNING -> "It's running"
        State.FINISHED -> "It's finished"
    }
    println(message)
    // It's running
}

{kotlin-runnable="true" id="kotlin-tour-enum-classes"}

Enum classes can have properties and member functions just like normal classes.

For example, let's say you're working with HTML and you want to create an enum class containing some colors. You want each color to have a property, let's call it rgb, that contains their RGB value as a hexadecimal. When creating the enum constants, you must initialize it with this property:

enum class Color(val rgb: Int) {
    RED(0xFF0000),
    GREEN(0x00FF00),
    BLUE(0x0000FF),
    YELLOW(0xFFFF00)
}

Kotlin stores hexadecimals as integers, so the rgb property has the Int type, not the String type.

{style="note"}

To add a member function to this class, separate it from the enum constants with a semicolon ;:

enum class Color(val rgb: Int) {
    RED(0xFF0000),
    GREEN(0x00FF00),
    BLUE(0x0000FF),
    YELLOW(0xFFFF00);

    fun containsRed() = (this.rgb and 0xFF0000 != 0)
}

fun main() {
    val red = Color.RED
    
    // Calls containsRed() function on enum constant
    println(red.containsRed())
    // true

    // Calls containsRed() function on enum constants via class names
    println(Color.BLUE.containsRed())
    // false
  
    println(Color.YELLOW.containsRed())
    // true
}

{kotlin-runnable="true" id="kotlin-tour-interface-enum-classes-members"}

In this example, the containsRed() member function accesses the value of the enum constant's rgb property using the this keyword and checks if the hexadecimal value contains FF as its first bits to return a boolean value.

For more information, see Enum classes.

Inline value classes

Sometimes in your code, you may want to create small objects from classes and use them only briefly. This approach can have a performance impact. Inline value classes are a special type of class that avoids this performance impact. However, they can only contain values.

To create an inline value class, use the value keyword and the @JvmInline annotation:

@JvmInline
value class Email

The @JvmInline annotation instructs Kotlin to optimize the code when it is compiled. To learn more, see Annotations.

{style="tip"}

An inline value class must have a single property initialized in the class header.

Let's say that you want to create a class that collects an email address:

// The address property is initialized in the class header.
@JvmInline
value class Email(val address: String)

fun sendEmail(email: Email) {
    println("Sending email to ${email.address}")
}

fun main() {
    val myEmail = Email("example@example.com")
    sendEmail(myEmail)
    // Sending email to example@example.com
}

{kotlin-runnable="true" id="kotlin-tour-inline-value-class"}

In the example:

  • Email is an inline value class that has one property in the class header: address.
  • The sendEmail() function accepts objects with type Email and prints a string to the standard output.
  • The main() function:
    • Creates an instance of the Email class called myEmail.
    • Calls the sendEmail() function on the myEmail object.

By using an inline value class, you make the class inlined and can use it directly in your code without creating an object. This can significantly reduce memory footprint and improve your code's runtime performance.

For more information about inline value classes, see Inline value classes.

Practice {completion-point="true"}

Exercise 1 {initial-collapse-state="collapsed" collapsible="true" id="special-classes-exercise-1"}

You manage a delivery service and need a way to track the status of packages. Create a sealed class called DeliveryStatus, containing data classes to represent the following statuses: Pending, InTransit, Delivered, Canceled. Complete the DeliveryStatus class declaration so that the code in the main() function runs successfully:

|---|---|

sealed class // Write your code here

fun printDeliveryStatus(status: DeliveryStatus) {
    when (status) {
        is DeliveryStatus.Pending -> {
            println("The package is pending pickup from ${status.sender}.")
        }
        is DeliveryStatus.InTransit -> {
            println("The package is in transit and expected to arrive by ${status.estimatedDeliveryDate}.")
        }
        is DeliveryStatus.Delivered -> {
            println("The package was delivered to ${status.recipient} on ${status.deliveryDate}.")
        }
        is DeliveryStatus.Canceled -> {
            println("The delivery was canceled due to: ${status.reason}.")
        }
    }
}

fun main() {
    val status1: DeliveryStatus = DeliveryStatus.Pending("Alice")
    val status2: DeliveryStatus = DeliveryStatus.InTransit("2024-11-20")
    val status3: DeliveryStatus = DeliveryStatus.Delivered("2024-11-18", "Bob")
    val status4: DeliveryStatus = DeliveryStatus.Canceled("Address not found")

    printDeliveryStatus(status1)
    // The package is pending pickup from Alice.
    printDeliveryStatus(status2)
    // The package is in transit and expected to arrive by 2024-11-20.
    printDeliveryStatus(status3)
    // The package was delivered to Bob on 2024-11-18.
    printDeliveryStatus(status4)
    // The delivery was canceled due to: Address not found.
}

{validate="false" kotlin-runnable="true" kotlin-min-compiler-version="1.3" id="kotlin-tour-special-classes-exercise-1"}

|---|---|

sealed class DeliveryStatus {
    data class Pending(val sender: String) : DeliveryStatus()
    data class InTransit(val estimatedDeliveryDate: String) : DeliveryStatus()
    data class Delivered(val deliveryDate: String, val recipient: String) : DeliveryStatus()
    data class Canceled(val reason: String) : DeliveryStatus()
}

fun printDeliveryStatus(status: DeliveryStatus) {
    when (status) {
        is DeliveryStatus.Pending -> {
            println("The package is pending pickup from ${status.sender}.")
        }
        is DeliveryStatus.InTransit -> {
            println("The package is in transit and expected to arrive by ${status.estimatedDeliveryDate}.")
        }
        is DeliveryStatus.Delivered -> {
            println("The package was delivered to ${status.recipient} on ${status.deliveryDate}.")
        }
        is DeliveryStatus.Canceled -> {
            println("The delivery was canceled due to: ${status.reason}.")
        }
    }
}

fun main() {
    val status1: DeliveryStatus = DeliveryStatus.Pending("Alice")
    val status2: DeliveryStatus = DeliveryStatus.InTransit("2024-11-20")
    val status3: DeliveryStatus = DeliveryStatus.Delivered("2024-11-18", "Bob")
    val status4: DeliveryStatus = DeliveryStatus.Canceled("Address not found")

    printDeliveryStatus(status1)
    // The package is pending pickup from Alice.
    printDeliveryStatus(status2)
    // The package is in transit and expected to arrive by 2024-11-20.
    printDeliveryStatus(status3)
    // The package was delivered to Bob on 2024-11-18.
    printDeliveryStatus(status4)
    // The delivery was canceled due to: Address not found.
}

{initial-collapse-state="collapsed" collapsible="true" collapsed-title="Example solution" id="kotlin-tour-special-classes-solution-1"}

Exercise 2 {initial-collapse-state="collapsed" collapsible="true" id="special-classes-exercise-2"}

In your program, you want to be able to handle different statuses and types of errors. You have a sealed class to capture the different statuses which are declared in data classes or objects. Complete the code below by creating an enum class called Problem that represents the different problem types: NETWORK, TIMEOUT, and UNKNOWN.

|---|---|

sealed class Status {
    data object Loading : Status()
    data class Error(val problem: Problem) : Status() {
        // Write your code here
    }

    data class OK(val data: List<String>) : Status()
}

fun handleStatus(status: Status) {
    when (status) {
        is Status.Loading -> println("Loading...")
        is Status.OK -> println("Data received: ${status.data}")
        is Status.Error -> when (status.problem) {
            Status.Error.Problem.NETWORK -> println("Network issue")
            Status.Error.Problem.TIMEOUT -> println("Request timed out")
            Status.Error.Problem.UNKNOWN -> println("Unknown error occurred")
        }
    }
}

fun main() {
    val status1: Status = Status.Error(Status.Error.Problem.NETWORK)
    val status2: Status = Status.OK(listOf("Data1", "Data2"))

    handleStatus(status1)
    // Network issue
    handleStatus(status2)
    // Data received: [Data1, Data2]
}

{validate="false" kotlin-runnable="true" kotlin-min-compiler-version="1.3" id="kotlin-tour-special-classes-exercise-2"}

|---|---|

sealed class Status {
    data object Loading : Status()
    data class Error(val problem: Problem) : Status() {
        enum class Problem {
            NETWORK,
            TIMEOUT,
            UNKNOWN
        }
    }

    data class OK(val data: List<String>) : Status()
}

fun handleStatus(status: Status) {
    when (status) {
        is Status.Loading -> println("Loading...")
        is Status.OK -> println("Data received: ${status.data}")
        is Status.Error -> when (status.problem) {
            Status.Error.Problem.NETWORK -> println("Network issue")
            Status.Error.Problem.TIMEOUT -> println("Request timed out")
            Status.Error.Problem.UNKNOWN -> println("Unknown error occurred")
        }
    }
}

fun main() {
    val status1: Status = Status.Error(Status.Error.Problem.NETWORK)
    val status2: Status = Status.OK(listOf("Data1", "Data2"))

    handleStatus(status1)
    // Network issue
    handleStatus(status2)
    // Data received: [Data1, Data2]
}

{initial-collapse-state="collapsed" collapsible="true" collapsed-title="Example solution" id="kotlin-tour-special-classes-solution-2"}

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