How to use sealed classes in Kotlin to model hierarchies?

Sealed classes are used to represent restricted inheritance hierarchies, where all possible subclasses are known at compile time.

Main features:

• All direct subclasses must be in the same file as the sealed class (before Kotlin 1.5). Starting from Kotlin 1.5, they can be in any file of the same compilation.
• Subclasses of a sealed class can be regular classes, data classes, object declarations, or other sealed classes.
• The constructor of a sealed class is private by default; other modifiers (public, protected, internal) are not allowed.
• They allow exhaustive analysis in when-expressions, which guarantees handling all possible cases and eliminates the need for an else branch.

Example of usage for modeling user interface state:

sealed class UiState {
    object Loading : UiState() // Loading state
    data class Success(val data: String) : UiState() // Successful data retrieval
    data class Error(val message: String) : UiState() // Error
}

Using in a when expression:

fun handleUiState(state: UiState): String {
    return when (state) {
        UiState.Loading -> "Loading data..."
        is UiState.Success -> "Data loaded successfully: ${state.data}"
        is UiState.Error -> "Error: ${state.message}"
    }
}

Advantages:

• Type safety: the compiler checks that all possible subclasses are handled.
• Readability: clearly shows what data or states are possible.
• Convenience: simplifies working with when expressions.