Sealed classes

Sealed classes and interfaces represent restricted class hierarchies that provide more control over inheritance. All direct subclasses of a sealed class are known at compile time. No other subclasses may appear after a module with the sealed class is compiled. For example, third-party clients can't extend your sealed class in their code. Thus, each instance of a sealed class has a type from a limited set that is known when this class is compiled.

The same works for sealed interfaces and their implementations: once a module with a sealed interface is compiled, no new implementations can appear.

In some sense, 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.

As an example, consider a library's API. It's likely to contain error classes to let the library users handle errors that it can throw. If the hierarchy of such error classes includes interfaces or abstract classes visible in the public API, then nothing prevents implementing or extending them in the client code. However, the library doesn't know about errors declared outside it, so it can't treat them consistently with its own classes. With a sealed hierarchy of error classes, library authors can be sure that they know all possible error types and no other ones can appear later.

To declare a sealed class or interface, put the sealed modifier before its name:

sealed interface Error sealed class IOError(): Error class FileReadError(val f: 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.

Constructors of sealed classes can have one of two visibilities: protected (by default) or private:

sealed class IOError { constructor() { /*...*/ } // protected by default private constructor(description: String): this() { /*...*/ } // private is OK // public constructor(code: Int): this() {} // Error: public and internal are not allowed } 

sealed interface Error // has implementations only in same package and module sealed class IOError(): Error // extended only in same package and module open class CustomError(): Error // can be extended wherever it's visible 

fun log(e: Error) = when(e) { is FileReadError -> { println("Error while reading file ${e.file}") } is DatabaseError -> { println("Error while reading from database ${e.source}") } RuntimeError -> { println("Runtime error") } // the `else` clause is not required because all the cases are covered }