How do sealed classes work in Kotlin?

Sealed classes in Kotlin are a powerful way to define a restricted class hierarchy. When you declare a class as sealed, you are essentially indicating that this class can have a limited number of subclasses, all of which must be declared within the same file. This feature is particularly useful for representing a fixed set of types in situations like representing various states, responses, or events.

By restricting the subclasses to only those defined in the same file, sealed classes enable exhaustive when statements. When you use a sealed class in a when expression, the compiler knows all possible types that can be present and can enforce that all cases are handled, either through specific cases or a fallback with else. This enhances type safety and reduces the risk of runtime exceptions due to unhandled cases.

In contrast, subclassing a sealed class anywhere in the project would violate the very purpose of having a restricted class hierarchy. Multiple inheritance is not enabled by sealed classes; instead, they are designed to avoid complexity in the inheritance model. While sealed classes can contain common functionalities like interfaces, they are distinct in their own right and do not function solely as interfaces. Thus, defining a restricted class hierarchy is the key attribute that sealed classes offer in Kotlin.