Careeroza

Interfaces

Defining an Interface

An interface is a contract: a list of functions and properties a class promises to provide. Kotlin interfaces can also contain default implementations. They cannot store state (no backing fields).

Example

interface Clickable {
    fun click()                                    // abstract
    fun showRipple() = println("Showing ripple")   // default implementation
}

class Button(val label: String) : Clickable {
    override fun click() = println("$label clicked")
}

fun main() {
    val b = Button("Pay Now")
    b.click()
    b.showRipple()
}

Output

Pay Now clicked
Showing ripple

Properties in Interfaces

Example

An interface property is either abstract or has a getter; implementing classes provide the value.

interface Identifiable {
    val id: String
    val shortId: String get() = id.take(6)   // computed from id
}

class Order(override val id: String, val amount: Int) : Identifiable

fun main() {
    val o = Order("ORD20261012XYZ", 899)
    println("${o.id} -> ${o.shortId}")
}

Output

ORD20261012XYZ -> ORD202

Exercise

Exercise: Create an interface Discountable with val price: Double and a default function finalPrice(percent: Int). Implement it in Shoe.

Show answer

interface Discountable {
    val price: Double
    fun finalPrice(percent: Int) = price - price * percent / 100
}

class Shoe(override val price: Double) : Discountable

fun main() {
    println(Shoe(2000.0).finalPrice(20)) // 1600.0
}