Kotlin Abstract Classes

Abstract classes are a fundamental concept in Kotlin's object-oriented programming paradigm. They serve as blueprints for other classes, providing a common structure and behavior while allowing for specialized implementations.

What are Abstract Classes?

An abstract class is a class that cannot be instantiated directly. It is designed to be subclassed, providing a common interface for a group of related classes. Abstract classes can contain both abstract and concrete methods, as well as properties.

Syntax and Usage

To declare an abstract class in Kotlin, use the abstract keyword before the class keyword:

abstract class Shape {
    abstract fun area(): Double
    abstract fun perimeter(): Double

    fun printDetails() {
        println("Area: ${area()}")
        println("Perimeter: ${perimeter()}")
    }
}

In this example, Shape is an abstract class with two abstract methods (area() and perimeter()) and one concrete method (printDetails()).

Implementing Abstract Classes

To use an abstract class, you must create a subclass that implements all the abstract methods:

class Circle(private val radius: Double) : Shape() {
    override fun area(): Double = Math.PI * radius * radius
    override fun perimeter(): Double = 2 * Math.PI * radius
}

In this example, Circle is a concrete class that extends the abstract Shape class and provides implementations for the abstract methods.

Key Features of Abstract Classes

• Cannot be instantiated directly
• May contain abstract and concrete methods
• Can have constructors and initialize properties
• Support Kotlin inheritance
• Can implement Kotlin interfaces

Abstract Classes vs Interfaces

While abstract classes and interfaces serve similar purposes, they have some key differences:

Abstract Classes	Interfaces
Can have constructors	Cannot have constructors
Support state (properties with backing fields)	Can only declare abstract properties or properties with default implementations
A class can extend only one abstract class	A class can implement multiple interfaces

Best Practices

• Use abstract classes when you want to provide a common base implementation for a group of related classes
• Prefer interfaces when you only need to define a contract without any implementation
• Keep abstract classes focused and cohesive, following the Single Responsibility Principle
• Use abstract classes to define template methods for algorithms with common steps but varying implementations

Abstract classes are a powerful tool in Kotlin for creating flexible and reusable code structures. They provide a balance between code reuse and specialization, making them an essential part of object-oriented design in Kotlin.

For more advanced topics related to abstract classes, explore Kotlin generics and Kotlin sealed classes.