Require an else branch.JSON serialization and generics in Kotlin. When expressions on expect sealed classes in the common code of multiplatform projects still 一个密封类是自身抽象的，它不能直接实例化并可以有抽象（ abstract）成员。Ĭonstructors of sealed classes can have one of two visibilities: protected (by default) orĬonstructor() To declare a sealed class or interface, put the sealed modifier before its name: sealed interface Error sealed class IOError(): ErrorĬlass FileReadError( val file: File): IOError()Ĭlass DatabaseError( val source: DataSource): IOError() Library authors can be sure that they know all possible error types and no other ones can appear later. With a sealed hierarchy of error classes, However, the library doesn't know about errorsĭeclared outside it, so it can't treat them consistently with its own classes. Then nothing prevents implementing or extending them in the client code. If the hierarchy of such error classes includes interfaces or abstract classes visible in the public API, It's likely to contain error classes to let the library users handle errors Of a sealed class can have multiple instances, each with its own state.Īs an example, consider a library's API. In some sense, sealed classes are similar to enum classes: the set of valuesįor an enum type is also restricted, but each enum constant exists only as a single instance, whereas a subclass The same works for sealed interfaces and their implementations: once a module with a sealed interface is compiled, Thus, each instance of a sealed class has a type from a limited set that is known when this class is compiled. For example, third-party clients can't extend your sealed class in their code. The module and package within which the sealed class is defined. Sealed classes and interfaces represent restricted class hierarchies that provide more control over inheritance.Īll direct subclasses of a sealed class are known at compile time.

Sealed class serializable kotlin