Polymorphism

Learn polymorphic serialization.

Polymorphic serialization

Polymorphic serialization turns different subclasses of a sealed class or interface into JSON. Each subclass is written with a type tag.

Example: @Serializable sealed class Shape. Subclasses @Serializable data class Circle(val radius: Double) : Shape() and @Serializable data class Square(val side: Double) : Shape(). In JSON it carries type info as {"type":"Circle","radius":5.0}. This is used to serialize different shapes in one list at once.

Polymorphism with sealed classes

A sealed class has subclasses known at compile time. It is ideal for polymorphic serialization.

Example: @Serializable sealed class Message. Subclasses: @Serializable data class TextMessage(val text: String) : Message() and @Serializable data class ImageMessage(val url: String) : Message(). A List<Message> holds both text and image messages. In JSON each element declares its subclass with a type field. This is useful in messaging apps.

Exercise Define a sealed class Shape

Define @Serializable sealed class Shape. Add a subclass @Serializable data class Circle(val radius: Double) : Shape(). Inside main, print println("Shape ready").

Show example solution

@Serializable
sealed class Shape

@Serializable
data class Circle(val radius: Double) : Shape()

fun main() {
    println("Shape ready")
}

Test yourself

Complete the sealed class definition

@Serializable
____ class Shape

Show answer

@Serializable
sealed class Shape

What is polymorphic serialization used for?
- To serialize subclasses with a type tag
- To prettyPrint JSON
- To compress JSON
- For thread safe serialization
Show answer

To serialize subclasses with a type tag — Polymorphic serialization writes subclasses with a type tag.
Match polymorphism parts

sealed class

Parent class
data class

Subclass
type

Type tag
@Serializable

Marker

Next lesson: Summary - kotlinx.serialization

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