sealed class
- Kotlin course
- Intermediate
- Lesson 24/116
- 5 min
Learn closed hierarchies with sealed class.
sealed class
A sealed class defines a closed hierarchy where the subtypes are limited and known. All subtypes must be declared in the same file (or in Kotlin 2 the same module).
Example: sealed class Status; object Loading : Status(); object Done : Status(). Status has only two subtypes: Loading and Done. This closure means the compiler knows all subtypes; when expressions can be exhaustive. Sealed classes are abstract; they cannot be instantiated directly.
Subtypes
Subtypes of a sealed class can be a class, a data class or an object. Each one inherits the sealed class.
Example: sealed class Status; object Loading : Status(); data class Error(val msg: String) : Status(). Here Loading is an object (single instance), Error is a data class (separate instance per error). Making a subtype an object expresses a single state; a data class expresses a state with fields. The sealed class itself is abstract.
Practice Define the Status sealed class
Define a sealed class named Status. Add two subtypes: object Loading : Status() and object Done : Status(). In main declare val s: Status = Done and print s with println.
Starter code
// Define the Status sealed class
// object Loading and object Done subtypes
// declare a Status variable in main, printSample solution
sealed class Status
object Loading : Status()
object Done : Status()
fun main() {
val s: Status = Done
println(s)
}Expected output
Done
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What does a <strong>sealed class</strong> provide?
- A closed hierarchy with known subtypes Correct answer
- The class can be instantiated directly
- Subclasses can be defined outside the file
- New subtypes can be added at runtime
Explanation: A sealed class defines a closed hierarchy with known subtypes.
2 Complete the sealed class definition
sealed ____ Status
object Loading : Status()
object Done : Status()sealed class Status
object Loading : Status()
object Done : Status()Explanation: sealed class declares a closed hierarchy.
3 Match the sealed class parts
| Term | Meaning |
|---|---|
sealed class Status |
Closed base class |
object Loading |
Loading state |
object Done |
Done state |
: Status() |
Inheritance and constructor |