What are the differences between Sealed Interface and Sealed Class in Kotlin?

In Kotlin, sealed class and sealed interface are used to restrict inheritance hierarchies, allowing the declaration of a fixed set of subclasses or implementations known at compile time.

Main differences:

• Sealed Class — a class that can have a limited number of subclasses. All subclasses must be declared in the same file as the sealed class (or within its scope). This is convenient for implementing hierarchies with a limited number of options, such as representing states or events.

• Sealed Interface — similar to a sealed class, but for interfaces. It allows declaring an interface with a limited number of implementations. This is useful when you need to describe behavior that different classes can implement but want to restrict their set.

Example:

sealed class Result {
    data class Success(val data: String) : Result()
    object Error : Result()
}

sealed interface Shape
class Circle(val radius: Double) : Shape
class Rectangle(val width: Double, val height: Double) : Shape

Thus, sealed class is suitable when a base implementation with states is needed, while sealed interface is important when the contract part with a limited number of implementations is required. Both help the compiler check the completeness of when expressions and ensure type safety.