Data classes, enums, and sealed types

Data class: a value made from properties

data class UserUi(val id: Long, val name: String)

Kotlin generates equals, hashCode, toString, componentN, and copy from primary-constructor properties.

val renamed = UserUi(1, "Ada").copy(name = "Grace")

copy is shallow: nested mutable objects are shared unless copied separately.

Enum: fixed named constants of one shape

enum class SortOrder { NAME, DATE, SIZE }

Use an enum when every option has the same conceptual shape. Enums may have properties and functions.

Sealed type: fixed variants with different data

sealed interface Result<out T> {
    data class Success<T>(val value: T) : Result<T>
    data class Failure(val message: String) : Result<Nothing>
    data object Loading : Result<Nothing>
}

Each variant may carry different data. An exhaustive when forces every state to be handled.

Open in Kotlin Playground↗

Expected: value, error, and loading descriptions.

Choosing the type

Need	Choose
Store comparable values	Data class
Fixed constants with the same shape	Enum
Closed alternatives with different data	Sealed class/interface
One shared no-data state	data object

✕ Contradictory Boolean state

data class UiState(val loading: Boolean, val error: Boolean, val data: String?)
// loading=true, error=true, data!=null is possible

✓ Model only valid alternatives

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

Interview-ready answer

Data classes model values, enums model a fixed set of same-shaped constants, and sealed types model a closed set of differently shaped states. Sealed types make when exhaustive, so adding a new state reveals every consumer that must handle it.