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 outside a module within which the sealed class is defined. 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:
A sealed class is abstract by itself, it cannot be instantiated directly and can have
Constructors of sealed classes can have one of two visibilities:
protected (by default) or
Location of direct subclasses
Direct subclasses of sealed classes and interfaces must be declared in the same package. They may be top-level or nested inside any number of other named classes, named interfaces, or named objects. Subclasses can have any visibility as long as they are compatible with normal inheritance rules in Kotlin.
Subclasses of sealed classes must have a proper qualified name. They can't be local nor anonymous objects.
These restrictions don't apply to indirect subclasses. If a direct subclass of a sealed class is not marked as sealed, it can be extended in any way that its modifiers allow:
Inheritance in multiplatform projects
There is one more inheritance restriction in multiplatform projects: direct subclasses of sealed classes must reside in the same source set. It applies to sealed classes without the
If a sealed class is declared as
expect in a common source set and have
actual implementations in platform source sets, both
actual versions can have subclasses in their source sets. Moreover, if you use a hierarchical structure, you can create subclasses in any source set between the
Sealed classes and when expression
The key benefit of using sealed classes comes into play when you use them in a
when expression. If it's possible to verify that the statement covers all cases, you don't need to add an
else clause to the statement. However, this works only if you use
when as an expression (using the result) and not as a statement: