What is the difference between an interface and an abstract class in Kotlin?

A class can implement multiple interfaces but extend only one abstract class. Interfaces cannot hold state (no backing fields), while abstract classes can. Use interfaces for capabilities and abstract classes for partial implementations with shared state.

Can Kotlin interfaces have default method bodies?

Yes. Interfaces can declare functions with bodies, so implementers inherit the default behaviour and may override if needed.

Intermediate 7 min readKotlin 2.0

Kotlin Interfaces — Contracts with Default Implementations

An interface defines a contract that classes can implement. Kotlin interfaces can have abstract functions and default implementations.

What You Will Learn

Declare interfaces
Implement an interface
Use default method bodies
Implement multiple interfaces
Interface properties

Declaring and Implementing

Interfaces use the interface keyword. Classes implement with : InterfaceName.

Basic Interface

kotlin
interface Printable {
    fun print()
    fun printTwice() {
        print()
        print()
    }
}

class Document(val content: String) : Printable {
    override fun print() {
        println(content)
    }
}

fun main() {
    val doc = Document("Hello, Interface!")
    doc.print()
    doc.printTwice()
}

Output

Hello, Interface! Hello, Interface! Hello, Interface!

print() is abstract — Document must implement it. printTwice() has a default implementation that calls print(). Document gets printTwice() for free.

Best Practice: Prefer interfaces over abstract classes for defining contracts. Kotlin classes can implement multiple interfaces but can only extend one class.

Multiple Interfaces

A class can implement many interfaces.

Multiple Interfaces

kotlin
interface Flyable {
    fun fly() = println("Flying")
}

interface Swimmable {
    fun swim() = println("Swimming")
}

class Duck : Flyable, Swimmable

fun main() {
    val duck = Duck()
    duck.fly()
    duck.swim()
}

Output

Flying Swimming

Duck implements both interfaces and inherits both default implementations. No override needed since no conflict exists.

Practice Exercise

Exercisemultiple choice

Can a Kotlin interface have a default method implementation?

Quick Quiz

How many interfaces can a Kotlin class implement?

Kotlin Interfaces — Contracts with Default Implementations

What You Will Learn

Declaring and Implementing

Basic Interface

Multiple Interfaces

Multiple Interfaces

Practice Exercise

Quick Quiz

Frequently Asked Questions

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