PHP 5.6.0 released

Astrazione delle classi

PHP 5 introduce classi e metodi astratti. Le classi definite come astratte non possono essere istanziate, e qualsiasi classe che contenga almeno un metodo astratto deve anch'essa essere astratta. I metodi definiti come astratti dichiarano unicamente la propria firma - non possono dichiarare un'implementazione.

Quando ereditati da una classe astratta, tutti i metodi segnalati come astratti nella dichiarazione della classe padre devono essere definiti dalla classe figlio; inoltre, questi metodi devono essere definiti con la stessa or a visibilità (o con una meno ristretta). Per esempio, se il metodo astratto è definito come protetto, l'implementazione della funzione può essere definita come protetta o publica, ma non privata. Inoltre le firme dei metodi devono corrispondere, cioè il tipo e il numero degli argomenti richiesti deve essere lo stesso. Per esempio, se la classe figlio definisce un argomento opzionale, mentre la firma del metodo astratto non lo fa, non ci sono conflitti nella firma. Questo vale anche per i costruttori a partire da PHP 5.4. Prima della 5.4 le firme dei costruttori possono essere differenti.

Example #1 Esempio di classe astratta

<?php
abstract class AbstractClass
{
    
// Forza la classe estendente a definire questo metodo
    
abstract protected function getValue();
    abstract protected function 
prefixValue($prefix);

    
// Metodi comuni
    
public function printOut() {
        print 
$this->getValue() . "\n";
    }
}

class 
ConcreteClass1 extends AbstractClass
{
    protected function 
getValue() {
        return 
"ConcreteClass1";
    }

    public function 
prefixValue($prefix) {
        return 
"{$prefix}ConcreteClass1";
    }
}

class 
ConcreteClass2 extends AbstractClass
{
    public function 
getValue() {
        return 
"ConcreteClass2";
    }

    public function 
prefixValue($prefix) {
        return 
"{$prefix}ConcreteClass2";
    }
}

$class1 = new ConcreteClass1;
$class1->printOut();
echo 
$class1->prefixValue('FOO_') ."\n";

$class2 = new ConcreteClass2;
$class2->printOut();
echo 
$class2->prefixValue('FOO_') ."\n";
?>

Il precedente esempio visualizzerà:

ConcreteClass1
FOO_ConcreteClass1
ConcreteClass2
FOO_ConcreteClass2

Example #2 Abstract class example

<?php
abstract class AbstractClass
{
    
// Il nostro metodo astratto ha solo bisogno di definire gli argomenti richiesti
    
abstract protected function prefixName($name);

}

class 
ConcreteClass extends AbstractClass
{

    
// La nostra classe figlio può definire argomenti opzionali non presenti nella firma del padre
    
public function prefixName($name$separator ".") {
        if (
$name == "Pacman") {
            
$prefix "Mr";
        } elseif (
$name == "Pacwoman") {
            
$prefix "Mrs";
        } else {
            
$prefix "";
        }
        return 
"{$prefix}{$separator} {$name}";
    }
}

$class = new ConcreteClass;
echo 
$class->prefixName("Pacman"), "\n";
echo 
$class->prefixName("Pacwoman"), "\n";
?>

Il precedente esempio visualizzerà:

Mr. Pacman
Mrs. Pacwoman

Codice vecchio che non ha classi definite dall'utente o funzioni dichiarate come 'abstract' dovrebbe poter essere eseguito senza necessità di modifiche.

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User Contributed Notes 15 notes

up
80
ironiridis at gmail dot com
6 years ago
Just one more time, in the simplest terms possible:

An Interface is like a protocol. It doesn't designate the behavior of the object; it designates how your code tells that object to act. An interface would be like the English Language: defining an interface defines how your code communicates with any object implementing that interface.

An interface is always an agreement or a promise. When a class says "I implement interface Y", it is saying "I promise to have the same public methods that any object with interface Y has".

On the other hand, an Abstract Class is like a partially built class. It is much like a document with blanks to fill in. It might be using English, but that isn't as important as the fact that some of the document is already written.

An abstract class is the foundation for another object. When a class says "I extend abstract class Y", it is saying "I use some methods or properties already defined in this other class named Y".

So, consider the following PHP:
<?php
class X implements Y { } // this is saying that "X" agrees to speak language "Y" with your code.

class X extends Y { } // this is saying that "X" is going to complete the partial class "Y".
?>

You would have your class implement a particular interface if you were distributing a class to be used by other people. The interface is an agreement to have a specific set of public methods for your class.

You would have your class extend an abstract class if you (or someone else) wrote a class that already had some methods written that you want to use in your new class.

These concepts, while easy to confuse, are specifically different and distinct. For all intents and purposes, if you're the only user of any of your classes, you don't need to implement interfaces.
up
29
joelhy
3 years ago
The documentation says: "It is not allowed to create an instance of a class that has been defined as abstract.". It only means you cannot initialize an object from an abstract class. Invoking static method of abstract class is still feasible. For example:
<?php
abstract class Foo
{
    static function
bar()
    {
        echo
"test\n";
    }
}

Foo::bar();
?>
up
28
a dot tsiaparas at watergate dot gr
3 years ago
Abstraction and interfaces are two very different tools. The are as close as hammers and drills. Abstract classes may have implemented methods, whereas interfaces have no implementation in themselves.

Abstract classes that declare all their methods as abstract are not interfaces with different names. One can implement multiple interfaces, but not extend multiple classes (or abstract classes).

The use of abstraction vs interfaces is problem specific and the choice is made during the design of software, not its implementation. In the same project you may as well offer an interface and a base (probably abstract) class as a reference that implements the interface. Why would you do that?

Let us assume that we want to build a system that calls different services, which in turn have actions. Normally, we could offer a method called execute that accepts the name of the action as a parameter and executes the action.

We want to make sure that classes can actually define their own ways of executing actions. So we create an interface IService that has the execute method. Well, in most of your cases, you will be copying and pasting the exact same code for execute.

We can create a reference implemention for a class named Service and implement the execute method. So, no more copying and pasting for your other classes! But what if you want to extend MySLLi?? You can implement the interface (copy-paste probably), and there you are, again with a service. Abstraction can be included in the class for initialisation code, which cannot be predefined for every class that you will write.

Hope this is not too mind-boggling and helps someone. Cheers,
Alexios Tsiaparas
up
8
mbajoras at gmail dot com
4 years ago
Here's an example that helped me with understanding abstract classes. It's just a very simple way of explaining it (in my opinion). Lets say we have the following code:

<?php
class Fruit {
    private
$color;
   
    public function
eat() {
       
//chew
   
}
   
    public function
setColor($c) {
       
$this->color = $c;
    }
}

class
Apple extends Fruit {
    public function
eat() {
       
//chew until core
   
}
}

class
Orange extends Fruit {
    public function
eat() {
       
//peel
        //chew
   
}
}
?>

Now I give you an apple and you eat it.

<?php
$apple
= new Apple();
$apple->eat();
?>

What does it taste like? It tastes like an apple. Now I give you a fruit.

<?php
$fruit
= new Fruit();
$fruit->eat();
?>

What does that taste like??? Well, it doesn't make much sense, so you shouldn't be able to do that. This is accomplished by making the Fruit class abstract as well as the eat method inside of it.

<?php
abstract class Fruit {
    private
$color;
   
    abstract public function
eat();
   
    public function
setColor($c) {
       
$this->color = $c;
    }
}
?>

Now just think about a Database class where MySQL and PostgreSQL extend it. Also, a note. An abstract class is just like an interface, but you can define methods in an abstract class whereas in an interface they are all abstract.
up
5
pete at surfaceeffect dot com
4 years ago
One fairly important difference between php's abstract functions and, say, Java, is that php does not specify the return type in any way - or indeed whether there has to be one.

<?php public abstract function square($number); ?>

could be implemented by...

<?php
public function square($number) {
   return
$number*$number;
}
?>

or

<?php
public function square($number) {
   print (
$number*$number);
}
?>

So you need to take care that incompatibilities don't arise due to not returning the right kind of value and this is not enforced in any way.
up
4
eeescalona
6 years ago
here is a real world example of abstract using:

a (abstract) person class
a student and an employee final class, which extends person class.

simple theory is that both student and employee is an extension of the person class.  the difference lies on which table the data is written on, and what other pre processing (ie mandatory field checking, type checking, etc.) needed before writing each of the classes.

codes:

<?php

abstract class person {
   
    abstract protected function
write_info();
   
    public
$LastName;
    public
$FirstName;
    public
$BirthDate;
   
    public function
get_Age($today=NULL){
       
//age computation function
   
}
}

final class
employee extends person{
    public
$EmployeeNumber;
    public
$DateHired;

    public function
write_info(){
        echo
"Writing ". $this->LastName . "'s info to emloyee dbase table";   
       
//ADD unique mandatory checking unique to EMPLOYEE ONLY
        //actual sql codes here
   
}
}

final class
student extends person{
    public
$StudentNumber;
    public
$CourseName;
   
    public function
write_info(){
        echo
"Writing ". $this->LastName . "'s info to student dbase table";
       
//ADD unique mandatory checking unique to STUDENT ONLY
        //actual sql codes here
   
}
}

///----------
$personA = new employee;
$personB = new student;

$personA->FirstName="Joe";
$personA->LastName="Sbody";

$personB->FirstName="Ben";
$personB->LastName="Dover";

$personA->write_info();
?>

OUTPUT:Writing Sbody's info to emloyee dbase table
up
1
nikola at petkanski dot com
6 months ago
Invoking static method of abstract class should be removed.

What interfaces are?
- A mean to ensure all implementation have the same methods implemented.

What is an abstract class?
- It is a interface that can also include some concrete methods.

Is it right for the developer to be able to invoke a static method of an interface?
- I think not.

The GoF teach us to rely on abstract classes and interfaces to hide differences between subclasses from clients.
- Interface defines an object’s use (protocol)
- Implementation defines particular policy

I don't think one should be able to call some abstract logic that is defined inside an abstract class, without even inheriting the class itself.
up
2
Cheese Doodle
4 years ago
There isn't really that much of a great hurdle in understanding these things, there really isn't.

If you're defining a new class that is abstract, it means that you can make some non-abstract functions that you can use to define the general underlying behavior of that class along side abstract ones.

In interfaces, you can't do that since functions defined therewithin cannot have a body.

Abstract functions you use for classes that must define more specific behavior when "extending" your class.

So for a crude example - define by your non-abstract functions how that particular object (which may be part of a larger class hierarchy) would store and process it's data in SQL, XML, etc.

Then define abstract functions which allow someone implementing that class to specifically manipulate the data that is to be stored. Then require a format which this data must be returned in, and then in your non-abstract functions call those functions on destruction, in normal runtime, and so on.

Again, non-abstract functions, or even another class could implement the finer points of ensuring that data is in the correct format, and so on, ad infinitum.

It isn't too much of a reach to say that if you used a normal class instead of an abstract class, then there isn't much intrinsic requirement between the two classes.

Assuming that you wanted the functions to use each-others functions and you'd need to use them specifically by name, you'd have to write some code which checked to see -- lamely using function_exists() and other lamery -- if that class has the function you require for interoperability, when you could avoid all possible confusion and headaches by simply using the right tool for the job.

And reading a decent OOP book.
up
1
joebert
7 years ago
I don't agree with jfkallens' last comparison between Abstract Classes & Object Interfaces completely.

In an Abstract Class, you can define how some methods work, where as in an Object Interface you can not.

An Object Interface is essentually nothing but a list of function names that a class must define if the class implements that interface.

An Abstract Class is essentually a prototype which hints towards what extending classes should be doing.
An Abstract Class can also be thought of as a Base Class that provides some basic functionality, & also defines a built-in Object Interface that all extending classes will implement.

So, an Object Interface is really a built-in part of an Abstract Class.
up
0
designbyjeeba at gmail dot com
3 years ago
Please be aware of the visibility of the parent fields. If the fields are private, then you are not going to see those fields in their childrens. Its basic OOP, but can be problematic sometimes.
up
0
bishop
4 years ago
Incidentally, abstract classes do not need to be base classes:

<?php
class Foo {
    public function
sneeze() { echo 'achoooo'; }
}

abstract class
Bar extends Foo {
    public abstract function
hiccup();
}

class
Baz extends Bar {
    public function
hiccup() { echo 'hiccup!'; }
}

$baz = new Baz();
$baz->sneeze();
$baz->hiccup();
?>
up
-1
nathan dot vorbei dot tech at gmail dot com
4 years ago
"additionally, these methods must be defined with the same (or a less restricted) visibility."

The words were not restricted in abstract class but also normal classes,
the method in child Class which overwrites the parent Class can also change the the visibility of the method to same or less restricted.
for example:
<?php
class ClassOne {
    protected static
$staticone = 'nathan';
    protected function
changestaticone() {
        return
self::$staticone = 'john';
    }
}

class
ClassTwo extends ClassOne {
    public function
changestaticone() {
        return
self::$staticone = 'Alexey';
    }
}
$classtwo = new ClassTwo();
echo
$classtwo->changestaticone();
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-3
oliver at ananit dot de
2 years ago
Abstract classes may have an final constructor, and sometime it makes sense to implement a class with a final constructor.

<?php
abstract class AbstractModel
{
   
//our models must use the default constuctor
       
public final function __construct(){}
    public function
inject($array){
        foreach(
array_keys(get_class_vars(get_called_class()))  as $property){
           
$this->$property = $array[$property];
        }
    }
}

class
ProductModel extends AbstractModel
{
    public
$name;
    public
$price;
    protected
$id;
   
    public function
getId(){return $this->id;}
}

class
Factory{
    private
$dataSource;
    public function
__consruct($dataSource){
       
$this->dataSource = $dataSource;
    }
   
    public function
get($class, $table, $filter, $orderby, $limit){
       
$result = array();
        foreach(
$datasource->fetchAssoc($table, $filter, $orderby, $limit) as $rawData){
           
$obj = new $class();//this can only work if ALL models have a default constructor
           
$obj->inject($rawData);
           
$result[] = $obj;
        }
        return
$result;
    }
}
?>

Note: This is a very simple example, and I am aware that there are other (better) ways to do this.
Oliver Anan
up
-4
sneakyimp at hotmail dot com
6 years ago
Ok...the docs are a bit vague when it comes to an abstract class extending another abstract class.  An abstract class that extends another abstract class doesn't need to define the abstract methods from the parent class.  In other words, this causes an error:

<?php
abstract class class1 {
  abstract public function
someFunc();
}
abstract class
class2 extends class1 {
  abstract public function
someFunc();
}
?>

Error: Fatal error: Can't inherit abstract function class1::someFunc() (previously declared abstract in class2) in /home/sneakyimp/public/chump.php on line 7

However this does not:

<?php
abstract class class1 {
  abstract public function
someFunc();
}
abstract class
class2 extends class1 {
}
?>

An abstract class that extends an abstract class can pass the buck to its child classes when it comes to implementing the abstract methods of its parent abstract class.
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-8
passerbyxp at gmail dot com
1 year ago
It's not quite clear in this document, so in case someone wonder, abstract method can be static:

<?php
abstract class FOO
{
    abstract static function
dump();
}
class
BAR extends FOO
{
    static function
dump(){var_dump("BAR");}
}
BAR::dump(); // string(3) "BAR"

class FAULT extends FOO
{
    function
pr(){var_export("WTF");}
}
// FATAL error
?>
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