When should I use a sealed class vs a regular class hierarchy?

Use a sealed class when you want exhaustive handling with when and you control all possible subclasses. Use a regular open class when external code should be allowed to subclass it.

Where must sealed class subclasses be declared?

All direct subclasses must live in the same file as the sealed class (or same module in newer Kotlin versions), keeping the hierarchy closed and known at compile time.

Intermediate 7 min readKotlin 2.0

Kotlin Sealed Classes — Restricted Class Hierarchies

A sealed class is a class with a fixed set of subclasses defined in the same package. This lets the compiler enforce exhaustive when expressions.

What You Will Learn

Declare sealed classes and their subclasses
Use sealed classes in when expressions
Model UI state with sealed classes
Difference between sealed class and enum

Declaring Sealed Classes

Use the sealed modifier. All direct subclasses must be in the same package.

Sealed Class for Result

kotlin
sealed class Result<out T>
data class Success<T>(val data: T) : Result<T>()
data class Error(val message: String) : Result<Nothing>()
object Loading : Result<Nothing>()

fun handle(result: Result<String>) {
    when (result) {
        is Success -> println("Data: ${result.data}")
        is Error   -> println("Error: ${result.message}")
        Loading    -> println("Loading...")
    }
}

fun main() {
    handle(Success("Hello Kotlin"))
    handle(Error("Network timeout"))
    handle(Loading)
}

Output

Data: Hello Kotlin Error: Network timeout Loading...

when is exhaustive — no else needed because the compiler knows all subclasses. is checks the type. Smart cast gives access to subclass properties.

Best Practice: Use sealed classes to model UI state: Loading, Success, Error. Use sealed interfaces for Kotlin 1.5+ (more flexible).

Sealed Class vs Enum

Both restrict the set of values, but sealed classes are more powerful.

Sealed vs Enum

kotlin
// Enum — each constant is the same type
enum class Direction { NORTH, SOUTH, EAST, WEST }

// Sealed — each subclass can have different properties
sealed class Event
data class Click(val x: Int, val y: Int) : Event()
data class KeyPress(val key: String) : Event()
object Dismiss : Event()

fun main() {
    val event: Event = Click(100, 200)
    when (event) {
        is Click -> println("Click at (${event.x}, ${event.y})")
        is KeyPress -> println("Key: ${event.key}")
        Dismiss -> println("Dismissed")
    }
}

Output

Click at (100, 200)

Sealed class subclasses can hold different data. Enum constants cannot. Use enums for simple sets of constants; sealed classes for types with distinct data.

Practice Exercise

Exercisemultiple choice

What advantage does a sealed class give in a when expression?

Quick Quiz

Where must sealed class subclasses be defined in Kotlin?

Kotlin Sealed Classes — Restricted Class Hierarchies

What You Will Learn

Declaring Sealed Classes

Sealed Class for Result

Sealed Class vs Enum

Sealed vs Enum

Practice Exercise

Quick Quiz

Frequently Asked Questions

Related Tutorials