34. Sealed Classes & Interfaces

A sealed type has a closed set of subtypes known at compile time. Because the compiler knows them all, a when that covers every case is exhaustive — no else branch required.

sealed interface Result

data class Success(val data: String) : Result
data class Failure(val error: String) : Result
data object Loading : Result

fun render(result: Result): String = when (result) {   // no else needed
    is Success -> "Data: ${result.data}"
    is Failure -> "Error: ${result.error}"
    Loading -> "Loading..."
}

fun main() {
    println(render(Success("hello")))
    println(render(Failure("oops")))
    println(render(Loading))
}

If you later add a new subtype, the when stops compiling until you handle it — a safety net that plain class hierarchies lack. A sealed class works the same way when you need shared state.

Running it:

$ kotlin run
Data: hello
Error: oops
Loading...

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