untung99.homes: PHP OOP Inheritance
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PHP – What is Inheritance?
Inheritance in OOP = When a class derives from another class.
The child class will inherit all the public and protected properties and
methods from the parent class. In addition, it can have its own properties and
methods.
An inherited class is defined by using the extends
keyword.
Let’s look at an example:
Example
class Fruit {
public
$name;
public $color;
public
function __construct($name, $color) {
$this->name = $name;
$this->color = $color;
}
public function intro() {
echo “The fruit is {$this->name}
and the color is {$this->color}.”;
}
}
// Strawberry is inherited from Fruit
class
Strawberry extends Fruit {
public
function message() {
echo “Am I a fruit or a
berry? “;
}
}
$strawberry = new Strawberry(“Strawberry”, “red”);
$strawberry->message();
$strawberry->intro();
?>
Try it Yourself »
Example Explained
The Strawberry class is inherited from the Fruit class.
This means that the Strawberry class can use the public $name and $color
properties as well as the public __construct() and intro() methods from the
Fruit class because of inheritance.
The Strawberry class also has its own method: message().
PHP – Inheritance and the Protected Access Modifier
In the previous chapter we learned that protected properties or methods can
be accessed within the
class and by classes derived from that class. What does that mean?
Let’s look at an example:
Example
class Fruit {
public
$name;
public $color;
public
function __construct($name, $color) {
$this->name = $name;
$this->color = $color;
}
protected function intro() {
echo “The fruit is {$this->name}
and the color is {$this->color}.”;
}
}
class
Strawberry extends Fruit {
public
function message() {
echo “Am I a fruit or a
berry? “;
}
}
// Try to call all three methods from outside class
$strawberry = new Strawberry(“Strawberry”, “red”);
// OK. __construct() is public
$strawberry->message(); // OK. message()
is public
$strawberry->intro(); // ERROR. intro()
is protected
?>
Try it Yourself »
In the example above we see that if we try to call a protected
method (intro()) from outside the class, we will receive an error. public
methods will work fine!
Let’s look at another example:
Example
class Fruit {
public $name;
public
$color;
public function __construct($name, $color) {
$this->name = $name;
$this->color = $color;
}
protected function intro() {
echo “The
fruit is {$this->name} and the color is {$this->color}.”;
}
}
class Strawberry extends Fruit {
public function message() {
echo “Am I a fruit or a berry? “;
// Call protected
method from within derived class – OK
$this ->
intro();
}
}
$strawberry = new Strawberry(“Strawberry”, “red”); // OK. __construct() is
public
$strawberry->message(); // OK. message() is
public and it calls intro() (which is protected) from within the
derived class
?>
Try it Yourself »
In the example above we see that all works fine! It is because we call the
protected
method (intro()) from inside the derived class.
PHP – Overriding Inherited Methods
Inherited methods can be overridden by redefining the methods (use the same
name) in the child class.
Look at the example below. The __construct() and intro() methods in the child
class (Strawberry) will override the __construct() and intro() methods in the
parent class (Fruit):
Example
class Fruit {
public
$name;
public $color;
public
function __construct($name, $color) {
$this->name = $name;
$this->color = $color;
}
public function intro() {
echo “The fruit is {$this->name}
and the color is {$this->color}.”;
}
}
class
Strawberry extends Fruit {
public $weight;
public
function __construct($name, $color, $weight) {
$this->name = $name;
$this->color = $color;
$this->weight = $weight;
}
public function intro() {
echo “The fruit is {$this->name}, the color is {$this->color},
and the weight is {$this->weight} gram.”;
}
}
$strawberry = new Strawberry(“Strawberry”, “red”,
50);
$strawberry->intro();
?>
Try it Yourself »
PHP – The final Keyword
The final
keyword can be used to prevent class inheritance or to prevent method overriding.
The following example shows how to prevent class inheritance:
Example
final class Fruit {
// some code
}
//
will result in error
class
Strawberry extends Fruit {
// some code
}
?>
Try it Yourself »
The following example shows how to prevent method overriding:
Example
class Fruit {
final public function intro() {
// some code
}
}
class
Strawberry extends Fruit {
//
will result in error
public function intro() {
// some code
}
}
?>
Try it Yourself »