Abstract Classes

Abstract Class

Abstract Class is a class that can't be instantiated (objects cannot be created) and may contain abstract methods.

It's supposed to represent something abstract and not specific.

Syntax:

abstract class ClassName { // code }

Basic Example

Let's look at the following example:

<?php abstract class Vehicle { public function getInsuranceCost($months){ return 0; } } class Car extends Vehicle { // Overwriting method public function getInsuranceCost($months) { return 25 * $months; } } class Bicycle extends Vehicle { } // $vehicle = new Vehicle(); $car = new Car(); echo "Car's Year Insurance: " . $car->getInsuranceCost(12) . " $\n"; $bicycle = new Bicycle(); echo "Bike's Year Insurance: " . $bicycle->getInsuranceCost(12) . " $\n";

Output:

Car's Year Insurance: 300 $ Bike's Year Insurance: 0 $

Explanation

• The abstract class Vehicle is defined at lines 2-7.
• The name of the abstract class is Vehicle. This is something indefinite and it's inherited by other classes.
• If we uncomment line 22 the code won't compile. That's because an abstract class cannot be instanced.

Abstract methods

Abstract methods can be defined only in abstract classes and have no body.

An abstract method in parent class must be overwritten in the child class.

Syntax:

abstract function funcName();

Example:

<?php abstract class Figure { abstract function calcArea(); } class Square extends Figure { private $side; public function __construct($sideValue) { $this->side = $sideValue; } // Implementation of abstract method: public function calcArea() { return $this->side * $this->side; } } class Rectangle extends Figure { private $width; private $height; public function __construct($widthValue, $heightValue) { $this->width = $widthValue; $this->height = $heightValue; } // Implementation of abstract method: public function calcArea() { return $this->width * $this->height; } } $square = new Square(2); $rectangle = new Rectangle(3, 2); echo "Square's area: " . $square->calcArea() . "\n"; echo "Rectangle's area: " . $rectangle->calcArea() . "\n";

Output:

Square's area: 4 Rectangle's area: 6

Explanation:

• At line 4 the abstract method calcArea is defined.
• Since calcArea is an abstract method, classes that inherit the abstract class Figure must implement the method. If not done then the code won't execute.
• At lines 17 - 20 and lines 35 - 38 the abstract method is implemented.