sealed class

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.

Exercise 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.

Show example solution

sealed class Status
object Loading : Status()
object Done : Status()

fun main() {
    val s: Status = Done
    println(s)
}

Test yourself

What does a <strong>sealed class</strong> provide?
- A closed hierarchy with known subtypes
- The class can be instantiated directly
- Subclasses can be defined outside the file
- New subtypes can be added at runtime
Show answer

A closed hierarchy with known subtypes — A sealed class defines a closed hierarchy with known subtypes.

Complete the sealed class definition

sealed ____ Status
object Loading : Status()
object Done : Status()

Show answer

sealed class Status
object Loading : Status()
object Done : Status()

Match the sealed class parts

sealed class Status

Closed base class
object Loading

Loading state
object Done

Done state
: Status()

Inheritance and constructor

Next lesson: when with sealed

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