Kotlin — Interfaces

Basic interface

interface Drawable {
    fun draw()
}

class Circle : Drawable {
    override fun draw() {
        println("Drawing circle")
    }
}

fun main() {
    val c = Circle()
    c.draw()
}

Interface with default implementations

interface Logger {
    fun log(message: String) {
        println("[LOG] $message")
    }

    fun error(message: String) {
        println("[ERROR] $message")
    }
}

class ConsoleLogger : Logger

fun main() {
    val logger = ConsoleLogger()
    logger.log("App started")
    logger.error("Something failed")
}

Multiple interfaces

interface Printable {
    fun print()
}

interface Loggable {
    fun log()
}

class Document : Printable, Loggable {
    override fun print() = println("Printing document")
    override fun log() = println("Logging document")
}

Property in interface

interface Named {
    val name: String
}

class Person(override val name: String, val age: Int) : Named

fun main() {
    val p = Person("Alice", 30)
    println(p.name) // Alice
}

Sealed interface

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

fun handle(result: Result) {
    when (result) {
        is Result.Success -> println("Data: ${result.data}")
        is Result.Error -> println("Error: ${result.message}")
    }
}

Mini Practice

Write Kotlin code that:

1. Defines an interface with a default method
2. Implements multiple interfaces in one class
3. Uses a sealed interface with when
4. Creates an interface with a property

Up Next

In the next lesson, you'll learn about Null Safety — Kotlin's null safety features.