Sealed classes for restricted hierarchies in Kotlin

Introduction

Sealed classes help you group related types in one place and control which classes can extend them. This makes your code safer and easier to understand.

When you want to represent a fixed set of types, like different states of a process.

When you want the compiler to check that you handle all possible cases in a when expression.

When you want to restrict class inheritance to a known set of subclasses.

When modeling choices or options that don't change often.

When you want clearer and safer code for handling different types of results.

Syntax

Kotlin

sealed class Shape {
    class Circle(val radius: Double) : Shape()
    class Rectangle(val width: Double, val height: Double) : Shape()
}

A sealed class can only be subclassed inside the same file where it is declared.

You can use a sealed class with a when expression to cover all cases without needing an else branch.

Examples

Defines a sealed class Result with two possible subclasses: Success and Error.

Kotlin

sealed class Result {
    class Success(val data: String) : Result()
    class Error(val error: Throwable) : Result()
}

Using a when expression to handle all subclasses of the sealed class Result.

Kotlin

fun handleResult(result: Result) = when(result) {
    is Result.Success -> println("Data: ${result.data}")
    is Result.Error -> println("Error: ${result.error.message}")
}

Sample Program

This program defines a sealed class TrafficLight with three fixed states. The action function returns what to do for each light. The main function prints the action for Yellow light.

Kotlin

sealed class TrafficLight {
    object Red : TrafficLight()
    object Yellow : TrafficLight()
    object Green : TrafficLight()
}

fun action(light: TrafficLight) = when(light) {
    TrafficLight.Red -> "Stop"
    TrafficLight.Yellow -> "Get Ready"
    TrafficLight.Green -> "Go"
}

fun main() {
    val currentLight = TrafficLight.Yellow
    println(action(currentLight))
}

Important Notes

Sealed classes improve safety by letting the compiler check all possible subclasses are handled.

You can use objects as subclasses when you don't need to store data, just represent a single value.

All subclasses must be declared in the same file as the sealed class.

Summary

Sealed classes restrict which classes can extend them, all in one file.

They help you write safer code by forcing you to handle all cases.

Use sealed classes to model fixed sets of related types clearly and simply.

Practice

1. What is the main purpose of using sealed classes in Kotlin?

easy

A. To create classes that cannot be instantiated

B. To allow any class in any file to inherit from it

C. To restrict which classes can inherit from it, all in the same file

D. To automatically generate data classes

Solution

1. Step 1: Understand sealed class inheritance rules

   Sealed classes restrict subclassing to the same file, limiting which classes can extend them.

2. Step 2: Compare options with sealed class purpose

   Only To restrict which classes can inherit from it, all in the same file correctly describes this restriction; others describe unrelated features.

3. Final Answer:

   To restrict which classes can inherit from it, all in the same file -> Option C

4. Quick Check:

   Sealed classes restrict inheritance = A [OK]

Hint: Sealed classes limit subclasses to one file only [OK]

Common Mistakes:

• Thinking sealed classes prevent instantiation
• Confusing sealed classes with abstract classes
• Believing sealed classes allow open inheritance

2. Which of the following is the correct syntax to declare a sealed class named Shape in Kotlin?

easy

A. abstract sealed class Shape

B. open sealed class Shape {}

C. sealed interface Shape {}

D. sealed class Shape {}

Solution

1. Step 1: Recall sealed class declaration syntax

   Sealed classes are declared with the keyword sealed class followed by the name and body.

2. Step 2: Check each option's correctness

   sealed class Shape {} matches the correct syntax. open sealed class Shape {} wrongly combines open and sealed. sealed interface Shape {} uses sealed interface which is invalid. abstract sealed class Shape {} wrongly combines abstract and sealed.

3. Final Answer:

   sealed class Shape {} -> Option D

4. Quick Check:

   Correct sealed class syntax = D [OK]

Hint: Use only 'sealed class ClassName {}' to declare sealed classes [OK]

Common Mistakes:

• Adding 'open' or 'abstract' keywords with sealed
• Using 'sealed interface' instead of 'sealed class'
• Forgetting curly braces after class name

3. Consider the following Kotlin code using sealed classes:

sealed class Result {
    data class Success(val data: String) : Result()
    object Failure : Result()
}

fun handle(result: Result) = when(result) {
    is Result.Success -> "Data: ${result.data}"
    is Result.Failure -> "Failed"
}

println(handle(Result.Success("OK")))

What is the output when this code runs?

medium

A. "Data: OK"

B. "Failed"

C. Compilation error due to missing else branch

D. Runtime exception

Solution

1. Step 1: Understand sealed class and when expression

   The sealed class Result has two subclasses: Success and Failure. The when expression covers both cases without else.

2. Step 2: Trace the function call with Success("OK")

   The argument is Result.Success("OK"), so the first branch matches, returning "Data: OK".

3. Final Answer:

   "Data: OK" -> Option A

4. Quick Check:

   Sealed class when covers all cases = "Data: OK" [OK]

Hint: Sealed classes allow exhaustive when without else [OK]

Common Mistakes:

• Thinking else branch is required for sealed classes
• Confusing Success and Failure outputs
• Expecting runtime errors from sealed classes

4. The following Kotlin code tries to compile but fails. What is the error?

sealed class Animal
class Dog : Animal()
class Cat : Animal()

Why does this code cause a compilation error?

medium

A. Sealed classes cannot have subclasses

B. Subclasses Dog and Cat must be declared in the same file as Animal

C. Dog and Cat must be declared as sealed classes too

D. Animal must be declared as an abstract class instead

Solution

1. Step 1: Recall sealed class subclass rules

   Sealed classes restrict subclasses to be declared in the same file.

2. Step 2: Identify the error cause

   Dog and Cat are subclasses but declared outside the sealed class file, causing compilation error.

3. Final Answer:

   Subclasses Dog and Cat must be declared in the same file as Animal -> Option B

4. Quick Check:

   Sealed subclasses must be in same file = B [OK]

Hint: All sealed subclasses must be in the same file [OK]

Common Mistakes:

• Thinking sealed classes forbid any subclassing
• Believing subclasses must be sealed too
• Confusing sealed with abstract class rules

5. You want to model a fixed set of user roles in Kotlin using sealed classes: Admin, Guest, and Member. Which approach correctly uses sealed classes to restrict roles and safely handle all cases?

hard

A. sealed class UserRole {
object Admin : UserRole()
object Guest : UserRole()
object Member : UserRole()
}

fun describe(role: UserRole) = when(role) {
is UserRole.Admin -> "Admin user"
is UserRole.Guest -> "Guest user"
is UserRole.Member -> "Member user"
}

B. open class UserRole
class Admin : UserRole()
class Guest : UserRole()
class Member : UserRole()

fun describe(role: UserRole) = when(role) {
is Admin -> "Admin user"
is Guest -> "Guest user"
is Member -> "Member user"
else -> "Unknown"
}

C. sealed interface UserRole {
object Admin : UserRole
object Guest : UserRole
object Member : UserRole
}

fun describe(role: UserRole) = when(role) {
UserRole.Admin -> "Admin user"
UserRole.Guest -> "Guest user"
UserRole.Member -> "Member user"
}

D. abstract class UserRole
object Admin : UserRole()
object Guest : UserRole()
object Member : UserRole()

fun describe(role: UserRole) = when(role) {
is Admin -> "Admin user"
is Guest -> "Guest user"
is Member -> "Member user"
}

Solution

1. Step 1: Use sealed class to restrict subclasses

   sealed class UserRole { object Admin : UserRole() object Guest : UserRole() object Member : UserRole() } fun describe(role: UserRole) = when(role) { is UserRole.Admin -> "Admin user" is UserRole.Guest -> "Guest user" is UserRole.Member -> "Member user" } declares a sealed class with all subclasses as objects inside the same file, restricting roles properly.

2. Step 2: Check safe handling with exhaustive when

   The when expression covers all subclasses without else, ensuring safe handling.

3. Final Answer:

   sealed class UserRole { object Admin : UserRole() object Guest : UserRole() object Member : UserRole() } fun describe(role: UserRole) = when(role) { is UserRole.Admin -> "Admin user" is UserRole.Guest -> "Guest user" is UserRole.Member -> "Member user" } -> Option A

4. Quick Check:

   Sealed class + exhaustive when = safe fixed roles [OK]

Hint: Use sealed class with all subclasses inside one file [OK]

Common Mistakes:

• Using open or abstract classes without sealing
• Missing exhaustive when branches
• Using sealed interface incorrectly