Sealed Classes in Kotlin

Analogy categoryAnalogies

Introduction

A sealed class is a special kind of abstract class that restricts which classes can inherit from it. All direct subclasses of a sealed class must be defined in the same file or module, so the compiler knows the complete set of possible subtypes at compile time. This makes sealed classes ideal for representing restricted states, such as the result of an operation that can only be Loading, Success, or Error. The sealed modifier marks the base class as having a closed hierarchy. Subclasses can be regular classes, data classes, or objects (for states without data). Because the compiler can see every subclass, a when expression over a sealed class value can be exhaustive without requiring an else branch, and the compiler will emit an error if a new subclass is added but not handled somewhere the hierarchy is checked.

🏏

Cricket analogy: A sealed class MatchResult with only Win, Loss, and Tie subclasses, all declared in the same file, lets the scorer's when block handle every possible outcome exhaustively without an else branch, since no fourth result can secretly exist.

Syntax

kotlin

sealed class Result {
    class Success(val data: String) : Result()
    class Error(val message: String) : Result()
    object Loading : Result()
}

Example

kotlin

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

fun main() {
    println(handle(Result.Success("42")))
    println(handle(Result.Error("Not found")))
    println(handle(Result.Loading))
}

Output

kotlin

Data: 42
Error: Not found
Loading...

Key Takeaways

Sealed classes restrict inheritance to a known, closed set of subclasses in the same file/module.
They enable exhaustive when expressions without an else branch.
Subclasses can be classes, data classes, or objects.
Ideal for modeling restricted states like Loading/Success/Error.
The compiler flags unhandled subclasses in when blocks, improving safety.