interface

Learn to implement interfaces.

interface

An interface defines a contract that implementing classes fill in. In Kotlin an interface can hold both bodyless methods and bodied default methods; however it cannot hold state.

Example: interface Drawable { fun draw(): String }. Classes implementing this interface must override fun draw(): String. A class can implement multiple interfaces: class Foo : A, B. Interfaces take no constructor parameters; this distinguishes them from abstract classes.

Implementing an interface

A class implements an interface with :. It must override the bodyless methods of the interface. Multiple interfaces are listed with commas.

Example: class Circle(val radius: Double) : Drawable { override fun draw() = "circle" }. Here Circle implements Drawable and also carries its own radius property. When implementing an interface, no constructor call is made (no parentheses); only the interface name is written. This is a different rule from class inheritance.

Exercise The Drawable interface and the Circle class

Define an interface named Drawable with a method fun draw(): String. Define a class named Circle with a primary constructor taking val radius: Double that implements the Drawable interface. Its override fun draw(): String returns "Circle drawn: radius $radius". In main create a Circle(3.0) instance and print the result of draw().

Show example solution

interface Drawable {
    fun draw(): String
}

class Circle(val radius: Double) : Drawable {
    override fun draw(): String {
        return "Circle drawn: radius $radius"
    }
}

fun main() {
    val c = Circle(3.0)
    println(c.draw())
}

Test yourself

Which of the following can a Kotlin interface not contain?
- A bodyless method
- A bodied default method
- An abstract method signature
- An instance field (state)
Show answer

An instance field (state) — Interfaces cannot hold state; they only define behavior.
Match the interface concepts

interface Drawable

Contract definition
fun draw(): String

Bodyless method
: Drawable

Implementation clause
override fun draw()

Filling in the class

Complete the interface implementation

class Circle(val radius: Double) : ____ {
    override fun draw() = "circle"
}

Show answer

class Circle(val radius: Double) : Drawable {
    override fun draw() = "circle"
}

Next lesson: Summary - Inheritance

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