Detailed information can be found in official document.

Sealed classes and interfaces | Kotlin

Sealed classes are similar to enum classes: the set of values for an enum type is also restricted, but each enum constant exists only as a single instance, whereas a subclass of a sealed class can have multiple instances, each with its own state.

• We can use object to create single instance in sealed class.

sealed interface Error
sealed class IOError(): Error
class FileReadError(val file: File): IOError()
class DatabaseError(val source: DataSource): IOError()
object RuntimeError : Error

A sealed class is abstract by itself, it cannot be instantiated directly and can have abstract members.

When Expression

If the statement covers all cases, we don't need to add an else clause to the statement.

Sealed Interface

• Kotlin enum classes can implement sealed interfaces
• a sealed interface is also a valid Kotlin interface
• Sealed interfaces allow multiple inheritance

// to define two new subhierarchies
sealed interface DinnerMenu
sealed interface LunchMenu

sealed class Menu {
    // resticting the Menu hieararchy to LaunchMenu and DinnerMenu
    data class PIZZA(val name: String, val size: String, val quantity: Int): Menu(), DinnerMenu
    data class BURGER(val quantity: Int, val size: String): Menu(), LunchMenu
    data class CHICKEN(val quantity: Int, val pieces: String): Menu(), LunchMenu
}