When Expressions & Sealed Classes: Pattern Matching

Exhaustive pattern matching with when and restricted class hierarchies

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The when expression replaces clunky switch statements with pattern matching, while sealed class defines closed type hierarchies verified by the compiler.

Sealed Class State Modeling

sealed class: Represents restricted class hierarchies where all subclasses are known at compile time. Ideal for UI State (Loading, Success, Error).

Exhaustive when: When using when on a sealed class, the compiler ensures every possible subclass is handled, eliminating the need for an else branch.

Smart Casts: Once Kotlin checks if (x is String) or when (state), it automatically casts x to String inside that block without explicit casting!

Managing Android Network State with Sealed Classes

Try It Yourself »

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

fun renderUi(state: UiState) {
    // Exhaustive pattern matching - compiler enforces all cases!
    when (state) {
        is UiState.Loading -> println("Displaying spinner...")
        is UiState.Success -> println("Rendering ${state.data.size} items.")
        is UiState.Error   -> println("Error alert: ${state.message}")
    }
}

Sealed classes guarantee that UI state transitions cannot have unhandled edge cases.

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Test Yourself With Exercise

+15 XP

What happens if you omit one of the subclasses in a `when` expression evaluating a `sealed class` in Kotlin?

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