abstract

Learn abstract classes with abstract.

abstract class

An abstract class cannot be instantiated directly and may contain abstract members that subclasses must fill in. An abstract method is declared without a body; a subclass must override it.

Example: abstract class Animal { abstract fun sound(): String }. You cannot write Animal() to instantiate it. A subclass must provide a body with override fun sound(): String. abstract classes are a stronger form of open; they are always open for inheritance.

Inheriting an abstract class

A subclass that inherits an abstract class must fill in all abstract methods with override. Otherwise the subclass must also be abstract.

Example: class Cat : Animal() { override fun sound() = "meow" }. Here Cat fills Animal's abstract sound. Cat is now a concrete class; you can write Cat() to instantiate it. Animal() cannot be instantiated; it is used only through a subclass.

Exercise The Animal abstract class and the Cat subclass

Define an abstract class named Animal with a primary constructor taking val name: String. Add an abstract fun sound(): String method. Define a subclass named Cat that inherits from Animal(name) and whose override fun sound(): String returns "$name meows". In main create a Cat instance and print the result of sound().

Show example solution

abstract class Animal(val name: String) {
    abstract fun sound(): String
}

class Cat(name: String) : Animal(name) {
    override fun sound(): String {
        return "$name meows"
    }
}

fun main() {
    val c = Cat("Whiskers")
    println(c.sound())
}

Test yourself

Complete the abstract method definition

abstract class Animal {
    ____ fun sound(): String
}

Show answer

abstract class Animal {
    abstract fun sound(): String
}

Match the abstract class concepts

abstract class Animal

Abstract base class
abstract fun sound()

Bodyless method
override fun sound()

Filling in subclass
class Cat : Animal()

Concrete subclass
<code>abstract class Animal</code> is defined. Which of the following cannot be done?
- Subclassing Animal
- Defining a concrete (bodied) method in Animal
- Instantiating Animal directly
- Defining an abstract method in Animal
Show answer

Instantiating Animal directly — An abstract class cannot be instantiated directly; it is used only through a subclass.

Next lesson: interface

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