Sealed classes and enums

Sealed Classes and Enums Deep Dive¶

Overview¶

Sealed classes and enum classes both model limited domains, but they solve different problems.

Core Concepts¶

Enum classes¶

Best for a fixed set of singleton constants.

enum class SortOrder {
    ASCENDING,
    DESCENDING
}

Sealed classes¶

Best for closed hierarchies where each case may carry different data.

sealed class UiState {
    object Loading : UiState()
    data class Success(val items: List<String>) : UiState()
    data class Error(val message: String) : UiState()
}

Internal Implementation¶

Enums create one instance per enum constant. Sealed classes define a restricted inheritance tree.

Key difference:

enum = fixed constants
sealed = fixed subtype set

That makes sealed classes much stronger for state/result/event modeling.

JVM / Compiler Behavior¶

Enums¶

Compile to Java-style enum classes with constant instances.

Sealed classes¶

Compile as regular classes/interfaces with metadata allowing the compiler to reason about exhaustiveness.

The JVM itself does not “understand” sealed behavior the same way Kotlin source does; much of the experience is compiler driven.

Code Examples¶

Exhaustive `when`¶

fun render(state: UiState): String = when (state) {
    UiState.Loading -> "loading"
    is UiState.Success -> state.items.joinToString()
    is UiState.Error -> state.message
}

Enum use case¶

enum class ThemeMode {
    LIGHT,
    DARK,
    SYSTEM
}

Common Interview Questions¶

Q: When should you choose sealed over enum? A: Connect Kotlin features to outcomes: safer APIs through nullability, clearer state modeling, and awareness of generated bytecode and allocation cost.
Q: Why is sealed useful for UI state? A: Connect Kotlin features to outcomes: safer APIs through nullability, clearer state modeling, and awareness of generated bytecode and allocation cost.
Q: Does sealed always mean no else in when? A: Connect Kotlin features to outcomes: safer APIs through nullability, clearer state modeling, and awareness of generated bytecode and allocation cost.

Production Considerations¶

Use enums for:

simple flags
static modes
limited status values

Use sealed classes for:

state machines
result wrappers
navigation outcomes
view state models

Performance Insights¶

The choice is usually more about modeling clarity than raw performance. But sealed class instances can allocate more frequently if used carelessly in high-frequency state updates.

Senior-Level Insights¶

The best modeling tool is the one that matches the domain:

enum for simple symbolic constants
sealed for structured closed-state systems

That distinction improves both correctness and maintainability.

Sealed classes and enums