PHP OOP - Abstract Classes
PHP - Abstract Classes and Methods
An abstract class is a class that contains at least one abstract method. An abstract method is a method that is declared, but not implemented in the abstract class. The implementation must be done in the child class(es).
The purpose of an abstract class is to enforce all derived classes (child classes) to implement the abstract method(s) declared in the parent class.
An abstract class or method is defined with the
abstract keyword.
Let's look at an example:
Example
<?php
// Abstract base class
abstract class Car {
public
$name;
// Non-abstract method
public
function __construct($name) {
$this->name = $name;
}
// Abstract method - forces child classes to implement it
abstract public function intro();
}
// Child class
that extends the abstract class
class
Audi extends Car {
public
function intro() {
return "German
quality! I'm an $this->name!";
}
}
// Child class that extends the abstract class
class
Citroen extends Car {
public
function intro() {
return "French
extravagance! I'm a $this->name!";
}
}
// Create objects of the child classes
$audi = new
audi("Audi");
echo $audi->intro();
echo "<br>";
$citroen = new citroen("Citroen");
echo $citroen->intro();
?>
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Example Explained
The Audi and Citroen classes are inherited from the abstract Car class. This means that the Audi and Citroen classes can use the public $name property as well as the public __construct() method from the Car class because of inheritance.
In addition, intro() is an abstract method that must be implemented in all the child classes.
Notes on Abstract Classes
When inheriting from an abstract class, the child class method must be defined with the same name, and the same or a less restricted access modifier. So, if the abstract method is defined as protected, the child class method must be defined as either protected or public, but not private. Also, the type and number of required arguments must be the same. However, the child classes may have optional arguments in addition.
So, when a child class is inherited from an abstract class, we have the following rules:
- The child class method must be defined with the same name and it redeclares the parent abstract method
- The child class method must be defined with the same or a less restricted access modifier
- The number of required arguments must be the same. However, the child class may have optional arguments in addition
PHP - Abstract Method with Argument
Let's look at another example where the abstract method has an argument:
Example
<?php
abstract class ParentClass {
// Abstract method
with an argument
abstract protected
function prefixName($name);
}
class ChildClass extends ParentClass {
public function prefixName($name) {
if ($name == "John Doe") {
$prefix = "Mr.";
} elseif ($name == "Jane Doe") {
$prefix =
"Mrs.";
} else {
$prefix = "";
}
return
"$prefix $name";
}
}
$class = new ChildClass;
echo $class->prefixName("John
Doe");
echo "<br>";
echo $class->prefixName("Jane Doe");
echo "<br>";
echo $class->prefixName("Baby Doe");
?>
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In the following example, the child class has two optional arguments that are not defined in the parent's abstract method:
Example
<?php
abstract class ParentClass {
// Abstract method
with an argument
abstract protected
function prefixName($name);
}
class ChildClass extends ParentClass {
// The child class may define optional arguments that are not in the parent's
abstract method
public function prefixName($name, $separator = ".",
$greet = "Dear") {
if ($name == "John Doe") {
$prefix = "Mr";
} elseif ($name == "Jane Doe") {
$prefix =
"Mrs";
} else {
$prefix = "";
}
return
"$greet $prefix$separator $name";
}
}
$class = new ChildClass;
echo $class->prefixName("John
Doe");
echo "<br>";
echo $class->prefixName("Jane Doe");
?>
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