Interface declaration and implementation in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an interface in Kotlin?

An interface in Kotlin is a contract that defines a set of functions and properties that a class can implement. It specifies what a class should do, but not how to do it.

beginner

How do you declare an interface in Kotlin?

Use the interface keyword followed by the interface name and curly braces. Inside, define abstract functions or properties. Example: <br>

interface Drivable { fun drive() }

beginner

How does a class implement an interface in Kotlin?

A class uses a colon <code>:</code> followed by the interface name to implement it. The class must provide concrete implementations for all interface functions. Example: <br><pre>class Car : Drivable { override fun drive() { println("Driving") } }</pre>

intermediate

Can interfaces in Kotlin have method implementations?

Yes, interfaces can have default method implementations. Classes implementing the interface can use these defaults or override them with their own versions.

intermediate

What happens if a class does not implement all interface methods?

The Kotlin compiler will show an error. The class must implement all abstract methods of the interface unless the class is declared abstract.

How do you declare an interface named Flyable in Kotlin?

A class Flyable { fun fly() }

B interface Flyable() { fly() }

C fun interface Flyable { fly() }

D interface Flyable { fun fly() }

The correct syntax uses the interface keyword followed by the name and curly braces with function declarations.

What keyword does a Kotlin class use to implement an interface?

A extends

B implements

C :

D inherits

Kotlin uses a colon : to indicate implementation or inheritance.

Can Kotlin interfaces contain properties?

A Yes, but they must be abstract or have default getters

B No, only functions

C Yes, with backing fields

D No, only constants

Interfaces can declare properties but they cannot have backing fields. They can have abstract properties or properties with default getters.

What must a class do when it implements an interface with abstract methods?

A Implement all abstract methods or be declared abstract

B Implement only some methods

C Nothing, methods are optional

D Override only default methods

The class must implement all abstract methods or be declared abstract itself.

If an interface method has a default implementation, can a class override it?

A No, default implementations cannot be changed

B Yes, the class can override it

C Only if the method is marked open

D Only if the class is abstract

Classes can override default interface methods to provide their own implementation.

Explain how to declare and implement an interface in Kotlin with a simple example.

Think about how you tell a class what it must do using an interface.

Describe what happens if a Kotlin class does not implement all abstract methods of an interface it declares to implement.

Consider what the compiler expects from a class that promises to follow an interface.