Kotlin - no_image

abstract class vs interface

An interface in Kotlin is a blueprint of a class. It describes a set of methods that a class agrees to implement. Unlike a class, an interface cannot hold state (it cannot store properties that are initialized). However, Kotlin interfaces can contain default method implementations. Interfaces are declared using the interface keyword¹, example:

interface Clickable {
    fun click()
    fun showOff() = println("I'm clickable!")
}

An abstract class, unlike an interface, can hold state (it can have properties that are initialized). Abstract classes are partially defined classes, methods, and properties that must be implemented by subclasses. Abstract classes are declared using the abstract keyword. Abstract methods and properties in an abstract class are not required to have an implementation in the abstract class itself but must be implemented in the subclasses², example:

abstract class Animated {
    abstract fun animate()
    open fun stopAnimating() {
        println("Stopped animating")
    }
    fun animateTwice() {
        animate()
        animate()
    }
}

The main differences are:

abstract class	interface
Can store state	Can’t store state
A class can inherit only from one abstract class	A class can implement multiple interfaces
Can provide the implementation of interface	Can’t provide the implementation of abstract class
Can extend another class and implement multiple interfaces	Can extend another interface only
Contains constructors	Does not contain constructors

Which should you use, abstract classes or interfaces?³

Consider using abstract classes if any of these statements apply to your situation:

• You want to share code among several closely related classes;
• You expect that classes that extend your abstract class have many common methods or fields, or require access modifiers other than public (such as protected and private).

Consider using interfaces if any of these statements apply to your situation:

• You expect that unrelated classes would implement your interface. For example, the interfaces Comparable and Cloneable are implemented by many unrelated classes;
• You want to specify the behavior of a particular data type, but not concerned about who implements its behavior;
• You want to take advantage of multiple inheritance of type.

Links

Interfaces and Abstract Classes

Abstract Methods and Classes

Further Reading

Why use Abstract classes (vs. interfaces)?

Abstract Classes and Interfaces in Kotlin: A Complete Guide

What is the difference between an interface and abstract class?

credit goes to @swayangjit