Interfaces and delegation

A school needs to print labels for different kinds of objects. A interface describes requirements a conforming type must provide, such as a property or method. It lets code depend on a capability without requiring every type to inherit from one common class.

An interface states properties or functions a type must provide and may supply suitable default implementations. A class can implement several interfaces. The by delegation syntax forwards an interface implementation to another object, making composition an alternative to rebuilding every operation.

Save the complete source as Main.kt and use the Kotlin/JVM environment. No separate input file is needed.

Kotlin/JVM · complete program

interface Labelled { val label: String }
data class Student(override val label: String) : Labelled
class Badge(private val owner: Labelled) : Labelled by owner
fun display(item: Labelled) { println(item.label) }
fun main() { display(Badge(Student("Diya"))) }

Expected output

Diya

Follow the execution

Student supplies the label property. Badge delegates that capability to its owner, so display reads Diya through the interface. No shared superclass between student data and badge presentation is required.

Try this in the Kotlin compiler by pasting the complete program.

Your task: Could a Room provide the same readable label capability?

Kotlin/JVM · complete program

interface Labelled { val label: String }
data class Student(override val label: String) : Labelled
class Badge(private val owner: Labelled) : Labelled by owner
fun display(item: Labelled) { println(item.label) }
fun main() { display(Badge(Student("Diya"))) }

Worked answer

Yes, by implementing Labelled and satisfying its property requirement. Display depends on the interface rather than one concrete record type.

Kotlin interfaces do not store ordinary backing fields for their properties. Resolve conflicting defaults deliberately. Delegation forwards behaviour; it does not deep-copy its delegate or automatically enforce thread safety.

Yes, by implementing Labelled and satisfying its property requirement. Display depends on the interface rather than one concrete record type. Kotlin interfaces do not store ordinary backing fields for their properties.

Further reading: official Kotlin documentation for this topic.