Sealed classes

Sealed classes restrict inheritance to a known set of subclasses in the same module—ideal for modeling closed hierarchies like UI states or network results, similar in spirit to C# discriminated unions with when exhaustiveness.

Sealed hierarchy

sealed class Result {
    data class Ok(val value: String) : Result()
    data class Err(val message: String) : Result()
}

fun describe(r: Result) = when (r) {
    is Result.Ok -> "OK: ${r.value}"
    is Result.Err -> "ERR: ${r.message}"
}

Subclasses must be direct—nested classes, objects, or data classes in the same file or module per visibility rules.

Important interview questions and answers

1. Q: Sealed vs enum?
   A: Enums are fixed constant sets with one instance each; sealed classes carry distinct data per subtype.
2. Q: Why sealed with when?
   A: Compiler can verify exhaustive when branches when all subclasses are known.

Self-check

1. What control flow pairs well with sealed types?
2. Can sealed subclasses live anywhere in the project?

Tip: Pair sealed hierarchies with exhaustive when—great for UI state and API result modeling.