Kotlin Fundamentals: Abstract Classes and Interfaces for Android

In Kotlin, abstract classes and interfaces are powerful tools for achieving abstraction and polymorphism, crucial concepts in building robust and maintainable Android applications. They allow you to define blueprints for classes, ensuring certain functionalities are implemented by subclasses.

Abstract Classes: The Incomplete Blueprint

An abstract class is a class that cannot be instantiated on its own. It serves as a base class for other classes and can contain both abstract members (methods or properties without implementation) and concrete members (methods or properties with implementation). Abstract members must be implemented by any non-abstract subclass.

Abstract classes provide a partial implementation and enforce structure for subclasses.

Think of an abstract class like a partially built house. It has some walls and a foundation (concrete members), but some rooms are still undefined (abstract members) and need to be completed by the person who moves in.

In Kotlin, you declare an abstract class using the abstract keyword. Subclasses inherit from abstract classes using the colon (:) syntax. If a subclass does not implement all abstract members of its parent abstract class, the subclass must also be declared as abstract.

What keyword is used to declare an abstract class in Kotlin?

The abstract keyword.

Interfaces: The Contract for Behavior

An interface in Kotlin defines a contract. It specifies a set of abstract methods and properties that implementing classes must provide. Unlike abstract classes, interfaces can only contain abstract members (though they can have default implementations for methods).

An interface is like a blueprint for a service. For example, a Printable interface might define a print() method. Any class that implements Printable (like a Document or a Report) guarantees that it has a print() method, even if the way they print is different. This allows you to treat different objects uniformly if they share a common interface.

📚

Text-based content

Library pages focus on text content

Classes can implement multiple interfaces, enabling a form of multiple inheritance of type. This is a key advantage over abstract classes, which can only be inherited from one at a time.

Can a Kotlin class implement multiple interfaces?

Yes, a Kotlin class can implement multiple interfaces.

Key Differences and Use Cases

Feature	Abstract Class	Interface
Instantiation	Cannot be instantiated	Cannot be instantiated
Inheritance	Single inheritance	Multiple inheritance (implementation)
Members	Abstract and concrete members	Primarily abstract members (can have default implementations)
State	Can hold state (properties)	Cannot hold state (properties must be abstract or have default implementations)
Use Case	When you want to share code and enforce a common structure among closely related classes	When you want to define a contract for behavior that unrelated classes can implement

In Android development, interfaces are frequently used to define callbacks, event handlers, and communication protocols between different components of your app.

Abstract Classes vs. Interfaces in Android Development

When building Android applications, understanding when to use an abstract class versus an interface is crucial for good design. Abstract classes are ideal when you have a base class with common functionality and state that many subclasses will share. Interfaces are better suited for defining capabilities or contracts that can be implemented by diverse classes, regardless of their inheritance hierarchy.

For instance, you might create an abstract

code

BaseViewModel

for your Android Architecture Components, providing common state management logic. Conversely, an

code

OnClickListener

interface in Android dictates that any class implementing it must provide a way to handle click events.

What is a common use case for interfaces in Android development?

Defining callbacks, event handlers, and communication protocols.

Learning Resources

Kotlin Abstract Classes - Official Documentation(documentation)

The official Kotlin documentation provides a clear explanation of abstract classes, their syntax, and how they function.

Kotlin Interfaces - Official Documentation(documentation)

Explore the official Kotlin documentation on interfaces, covering their definition, properties, and methods with default implementations.

Abstract Classes vs Interfaces in Kotlin - Tutorial(tutorial)

This tutorial breaks down the key differences between abstract classes and interfaces in Kotlin with practical examples.

Kotlin Programming: The Complete Developer's Guide - Abstract Classes(video)

A comprehensive course that often covers abstract classes and interfaces within its Kotlin fundamentals section, providing video explanations.

Understanding Kotlin Interfaces - Blog Post(blog)

A blog post detailing the nuances of Kotlin interfaces and their practical application in software development.

Kotlin Abstract Classes Explained - Medium(blog)

This Medium article offers a clear and concise explanation of abstract classes in Kotlin, suitable for beginners.

Android Development with Kotlin - Interfaces for Callbacks(documentation)

While not solely focused on interfaces, the official Android developer site for Kotlin often demonstrates their use in common patterns like callbacks.

Polymorphism in Kotlin - Tutorialspoint(tutorial)

Learn about polymorphism in Kotlin, a concept closely tied to abstract classes and interfaces, which enables treating objects of different classes in a uniform way.

Kotlin Abstract Class vs Interface - Stack Overflow Discussion(wikipedia)

A community discussion on Stack Overflow that delves into the practical differences and best practices for using abstract classes and interfaces in Kotlin.

Effective Kotlin: Item 16: Favor interfaces for type declarations(blog)

An insightful article from the 'Effective Kotlin' series discussing the advantages of using interfaces for defining types.