Best Kotlin Interface Guides to Buy in October 2025
Kotlin In-Depth: A Guide to a Multipurpose Programming Language for Server-Side, Front-End, Android, and Multiplatform Mobile (English Edition)
Kotlin in Action, Second Edition
Head First Kotlin: A Brain-Friendly Guide
Head First Android Development: A Learner's Guide to Building Android Apps with Kotlin
Kotlin Programming: Learning Guide Covering the Essentials and Advancing to Complex Concepts
Kotlin Programming: The Big Nerd Ranch Guide (Big Nerd Ranch Guides)
Android Programming with Kotlin for Beginners: Build Android apps starting from zero programming experience with the new Kotlin programming language
Functional Programming in Kotlin
Kotlin Design Patterns and Best Practices: Elevate your Kotlin skills with classical and modern design patterns, coroutines, and microservices
Kotlin from Scratch: A Project-Based Introduction for the Intrepid Programmer
To implement interfaces in Kotlin, you can follow these steps:
- Define an interface by using the interface keyword followed by the interface name. This interface can contain abstract properties, abstract methods, or both.
- In a class where you want to implement the interface, use the : symbol followed by the interface name to indicate that you are implementing that interface.
- Implement all the abstract properties and methods defined in the interface. You must provide an implementation for every abstract property or method declared in the interface.
- If required, you can also override default properties or methods defined in the interface within the implementing class.
- An implementing class can implement multiple interfaces by separating them with commas in the : syntax.
Remember that you must provide complete implementations for all abstract properties and methods in the interface when implementing it in a class. Otherwise, the compiler will give an error.
Using interfaces allows you to achieve abstraction and define contracts that classes must adhere to. It helps in creating loosely coupled code and enables polymorphism by treating objects of different classes that implement the same interface in a similar way.
That's how you can implement interfaces in Kotlin.
What are interface delegation and implementation delegation in Kotlin?
Interface delegation and implementation delegation are two approaches to implementing delegation in Kotlin.
- Interface Delegation: In this approach, you define an interface and a delegate class that implements the interface. The delegating class then delegates the implementation of the interface methods to the delegate class.
For example, consider an interface called Printer with a method print(). You can create a delegate class called RealPrinter that implements the Printer interface with its own implementation of the print() method. Then, you can create a delegating class called VirtualPrinter that holds an instance of RealPrinter and delegates the print() method to it.
interface Printer { fun print() }
class RealPrinter : Printer { override fun print() { println("Printing...") } }
class VirtualPrinter(private val printer: Printer) : Printer by printer
fun main() { val realPrinter = RealPrinter() val virtualPrinter = VirtualPrinter(realPrinter) virtualPrinter.print() // Delegates to RealPrinter's implementation }
In the above example, the VirtualPrinter delegates the implementation of the print() method to the RealPrinter using the by keyword. So, when virtualPrinter.print() is called, it invokes the print() method of the RealPrinter class.
- Implementation Delegation: In this approach, instead of defining an interface and a delegate class, you directly delegate the implementation of certain methods to another class by implementing the required interface.
Using the same example as above, instead of creating the RealPrinter class, you can directly implement the Printer interface in the VirtualPrinter class and delegate the print() method to an instance of Printer.
interface Printer { fun print() }
class VirtualPrinter(private val printer: Printer) : Printer { override fun print() { printer.print() } }
fun main() { val realPrinter = object : Printer { override fun print() { println("Printing...") } } val virtualPrinter = VirtualPrinter(realPrinter) virtualPrinter.print() // Delegates to custom implementation of print() }
In this case, the VirtualPrinter class implements the Printer interface and delegates the implementation of the print() method to the Printer instance passed in the constructor.
Both approaches provide a way to implement delegation in Kotlin, and the choice between them depends on the specific use case and requirements.
What is a marker interface in Kotlin?
A Marker interface in Kotlin is an interface that does not declare any methods or properties. Its role is to serve as a "marker" or "tag" for a specific purpose or attribute. By implementing a marker interface, a class indicates that it has a certain characteristic or behavior without adding any functionality.
Marker interfaces are commonly used in Kotlin and other languages to identify certain types or enable certain behavior. For example, the Serializable interface in Java serves as a marker interface to indicate that a class can be serialized and deserialized.
In Kotlin, you can define a marker interface using the interface keyword without declaring any members:
interface MarkerInterface
Classes implementing this interface can then be identified by the marker interface, allowing for specific actions or behavior to be taken.
What are companion objects in Kotlin interfaces?
In Kotlin, companion objects are used to define static members of a class or interface. When it comes to interfaces, companion objects can be declared inside the interface itself in order to specify static members that are associated with the interface.
Here is an example of using a companion object in a Kotlin interface:
interface ExampleInterface { fun printMessage()
companion object {
fun showInfo() {
println("This is an example companion object inside an interface.")
}
}
}
In the above code, the interface ExampleInterface declares a function printMessage(). Additionally, it includes a companion object that defines a static function showInfo(). The companion object can be referred to using the interface name, like ExampleInterface.showInfo().
Companion objects provide a way to define static members within an interface, which allows sharing common functionalities across classes that implement the interface.
What are functional interfaces in Kotlin?
In Kotlin, functional interfaces are interfaces that have only one abstract method. These interfaces are used to define lambda expressions or anonymous functions.
In other words, functional interfaces define the contract for a single function. They can have multiple default method implementations, but only one abstract method. This makes it possible to use lambda expressions or anonymous functions as an implementation for that single abstract method.
Kotlin provides the @FunctionalInterface annotation for marking an interface as a functional interface. However, this annotation is optional and not strictly enforced by the compiler. It's primarily used for documentation purposes to indicate that the interface is intended to be used as a functional interface.
Functional interfaces play a significant role in the functional programming style supported by Kotlin, allowing the use of lambda expressions and functional programming constructs like higher-order functions and extension functions.