Kotlin Data & Sealed Classes Cheat Sheet: Copy, When & Exhaustive Types

Data classes hold values: the compiler writes equals, hashCode, toString, copy, and destructuring for constructor properties. Sealed classes (and sealed interfaces) restrict subtypes so when is exhaustive — a better UI/network state machine than boolean pairs like isLoading + error.

This Kotlin cheat sheet is written for people searching for a fast, accurate reference: students, junior developers, and teams shipping production software. Pin it, copy the snippets, and come back when syntax slips.

Quick reference

• data class: data class User(val id: Int, val email: String) — val properties in the primary constructor.
• copy: user.copy(email = "new@x.com") — immutable updates.
• equals: Structural for data classes. Two copies with the same fields are equal.
• Destructure: val (id, email) = user follows constructor order.
• sealed class: Subtypes in the same compilation unit (relaxed in later Kotlin). Cannot construct the base.
• sealed interface: Same exhaustiveness, multiple inheritance of type.
• when: No else required when every sealed subtype is handled.
• enum vs sealed: Enum = fixed constants. Sealed = each subtype can carry different fields.

Copy-paste examples

UiState as a sealed class

Each screen state is a type. The UI when will not compile if you add a new state and forget a branch.

data class Project(val id: Int, val title: String)

sealed interface UiState {
    data object Loading : UiState
    data class Ready(val project: Project) : UiState
    data class Error(val message: String) : UiState
}

fun label(state: UiState): String = when (state) {
    UiState.Loading -> "Loading"
    is UiState.Ready -> state.project.title
    is UiState.Error -> state.message
}

Common mistakes

• Putting mutable var lists inside data classes and mutating them so equals lies.
• Using data classes for entities with identity where you only wanted a bag of fields in a Map.
• Open class hierarchies instead of sealed when the set of types is closed.
• copy() on a huge object graph as a substitute for a real reducer — still shallow on nested refs.

FAQ

1. Can a data class extend another data class? No. They can implement interfaces. Nest them or extract shared interfaces.

1. data object? A singleton object with a sane toString/equals, useful as a sealed state with no fields (Loading).

1. Why is BLOGPH0 shallow? It copies constructor args. If a property is a mutable list, both copies share it. Prefer immutable lists.

Related Kotlin cheat sheets

• Kotlin Cheat Sheet 2026
• Kotlin Android Cheat Sheet
• Kotlin Coroutines Cheat Sheet

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