🔌 Kotlin Interfaces – Complete Guide with Examples (2025)

Understand how interfaces work in Kotlin, how they differ from abstract classes, and how to implement multiple interfaces with real code examples.

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🚀 What is an Interface in Kotlin?

An interface in Kotlin defines a contract that classes can implement. It can contain:

• Abstract methods (without implementation)
• Concrete methods (with default implementation) — from Kotlin 1.1+
• Properties (abstract or with getters)

📌 Use interfaces when you want to define capabilities that can be shared across unrelated classes.

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🧱 Kotlin Interface Syntax

interface Drivable {
    fun drive()
    fun stop() {
        println("Vehicle stopped.")
    }
}

• drive() is abstract.
• stop() has a default implementation.

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🚗 Implementing an Interface

class Car : Drivable {
    override fun drive() {
        println("Driving the car...")
    }
}

fun main() {
    val myCar = Car()
    myCar.drive()      // Output: Driving the car...
    myCar.stop()       // Output: Vehicle stopped.
}

✅ The class must override all abstract methods in the interface.

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🔁 Multiple Interfaces

Kotlin supports multiple interface inheritance, enabling classes to implement multiple behaviors:

interface Drivable {
    fun drive()
}

interface Flyable {
    fun fly()
}

class FlyingCar : Drivable, Flyable {
    override fun drive() = println("FlyingCar is driving.")
    override fun fly() = println("FlyingCar is flying.")
}

🧠 This allows composition of behaviors, similar to mixins.

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🧮 Interfaces with Properties

interface Shape {
    val name: String
    fun draw()
}

class Circle : Shape {
    override val name = "Circle"
    override fun draw() = println("Drawing a circle.")
}

• Properties can be declared in interfaces.
• Must be overridden or have a default getter.

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🤺 Interface Inheritance

Interfaces can extend other interfaces:

interface Machine {
    fun operate()
}

interface SmartMachine : Machine {
    fun selfCheck()
}

A class implementing SmartMachine must also implement operate().

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🧭 Interface vs Abstract Class – Quick Comparison

Feature	Interface	Abstract Class
Multiple inheritance	✅ Yes	❌ No
Method implementation	✅ Yes (default methods)	✅ Yes
Properties	✅ Abstract or with getters	✅ Full property support
Constructors	❌ Not allowed	✅ Allowed
Use case	Behavior contracts	Base classes with logic

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📋 Summary

Concept	Description
interface	Defines a contract of abstract + optional methods
default method	Function with implementation (from Kotlin 1.1+)
multiple interfaces	Kotlin supports implementing multiple interfaces
interface inheritance	Interfaces can extend other interfaces
properties	Can be abstract or have custom getter

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❓ FAQ – Kotlin Interfaces

🔹 Q1: Can an interface have a constructor?

A: No. Interfaces in Kotlin cannot have constructors or state (fields).

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🔹 Q2: Can you inherit from multiple interfaces in Kotlin?

A: Yes. Kotlin supports multiple interface inheritance.

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🔹 Q3: Can you override default methods in interfaces?

A: Yes. Classes can override default methods provided in interfaces.

interface Greetable {
    fun greet() = println("Hello!")
}

class User : Greetable {
    override fun greet() = println("Hi there!")
}

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🔹 Q4: Can interfaces contain properties?

A: Yes, but:

• They must be abstract, or
• Have only getter implementations.

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🔹 Q5: Is interface the same as abstract class?

A: No. Use interface for defining capabilities and abstract class for shared code base and inheritance.

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