PHP 5.6.0 released

Final Keyword

PHP 5 introduces the final keyword, which prevents child classes from overriding a method by prefixing the definition with final. If the class itself is being defined final then it cannot be extended.

Example #1 Final methods example

<?php
class BaseClass {
   public function 
test() {
       echo 
"BaseClass::test() called\n";
   }
   
   final public function 
moreTesting() {
       echo 
"BaseClass::moreTesting() called\n";
   }
}

class 
ChildClass extends BaseClass {
   public function 
moreTesting() {
       echo 
"ChildClass::moreTesting() called\n";
   }
}
// Results in Fatal error: Cannot override final method BaseClass::moreTesting()
?>

Example #2 Final class example

<?php
final class BaseClass {
   public function 
test() {
       echo 
"BaseClass::test() called\n";
   }

   
// Here it doesn't matter if you specify the function as final or not
   
final public function moreTesting() {
       echo 
"BaseClass::moreTesting() called\n";
   }
}

class 
ChildClass extends BaseClass {
}
// Results in Fatal error: Class ChildClass may not inherit from final class (BaseClass)
?>

Nota: Properties cannot be declared final, only classes and methods may be declared as final.

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User Contributed Notes 11 notes

up
27
jriddy at gmail dot com
5 years ago
Note for Java developers: the 'final' keyword is not used for class constants in PHP. We use the keyword 'const'.

http://php.net/manual/en/language.oop5.constants.php
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10
penartur at yandex dot ru
7 years ago
Note that you cannot ovverride final methods even if they are defined as private in parent class.
Thus, the following example:
<?php
class parentClass {
    final private function
someMethod() { }
}
class
childClass extends parentClass {
    private function
someMethod() { }
}
?>
dies with error "Fatal error: Cannot override final method parentClass::someMethod() in ***.php on line 7"

Such behaviour looks slight unexpected because in child class we cannot know, which private methods exists in a parent class and vice versa.

So, remember that if you defined a private final method, you cannot place method with the same name in child class.
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5
santoshjoshi2003 at yahoo dot co dot in
5 years ago
The use of final keyword is just like that occurs in Java
In java final has three uses
    1) prevent class Inheritance
    2) prevent method overriding or redifination of
        method in subclass
    3) and to declare constants
But the third point seems to be missing from the PHP
I guess, as i am a java developer Currently gaining competence in PHP
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2
someone dot else at elsewhere dot net
7 months ago
@thomas at somewhere dot com

The 'final' keyword is extremely useful.  Inheritance is also useful, but can be abused and becomes problematic in large applications.  If you ever come across a finalized class or method that you wish to extend, write a decorator instead.

<?php
final class Foo
{
    public
method doFoo()
    {
       
// do something useful and return a result
   
}
}

final class
FooDecorator
{
    private
$foo;
   
    public function
__construct(Foo $foo)
    {
       
$this->foo = $foo;
    }
   
    public function
doFoo()
    {
         
$result = $this->foo->doFoo();
         
// ... customize result ...
         
return $result;
    }
}
?>
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5
Anonymous
3 years ago
The behaviour of FINAL is not as serious as you may think. A little explample:
<?php
class A {
     final private function
method(){}    
}

class
B extends A {
    private function
method(){}
}
?>

Normally you would expect some of the following will happen:
- An error that final and private keyword cannot be used together
- No error as the private visibility says, that a method/var/etc. is only visible within the same class

But what happens is PHP is a little curios: "Cannot override final method A::method()"

So its possible to deny method names in subclasses! Don't know if this is  a good behavior, but maybe its useful for your purpose.
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2
slorenzo at clug dot org dot ve
6 years ago
<?php
class parentClass {
    public function
someMethod() { }
}
class
childClass extends parentClass {
    public final function
someMethod() { } //override parent function
}

$class = new childClass;
$class->someMethod(); //call the override function in chield class
?>
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1
Baldurien
3 years ago
"Note for Java developers: the 'final' keyword is not used for class constants in PHP. We use the keyword 'const'."

http://php.net/manual/en/language.oop5.constants.php

This is more or less true, regardless of the fact that constant (being defined at class level or not) in PHP are only scalar (int, string, etc) while in Java they may be pure object (ex: java.awat.Color.BLACK). The only possible solution of having such kind of constant is :

<?php
class Bar {...}
class
Foo {
  public static
$FOOBAR;

  static function
__init() {
    static
$init = false;
    if (
$init) throw new Exception('Constants were already initialized');
   
self::$FOOBAR = new Bar();
   
$init = true;
  }
}
Foo::__init();
?>
That said, perhaps it is useless unless PHP automatically calls the __init() method.

However, one alternative that could be done in certain case is this :

<?php
function __autoload($className) {
  ... require
the file where the class is ...
  if (
interface_exists($className, false)) return;
  if (
class_exists($className, false)) {
   
$rc = new ReflectionClass($className);
    if (!
$rc->hasMethod('__init')) return;
   
$m = $rc->getMethod('__init');
    if (!(
$m->isStatic() && $m->isPrivate())) {
      throw new
Exception($className . ' __init() method must be private and static !');
    }
   
$m->invoke(null);
    return;
  }
  throw new
Exception('Class or interface not found ' . $className);
}
?>

This can only work when one class is defined per file, since we are assured that __autoload() will be called to load the file containing the class.

eg:

test2.php:
<?php
class B {
public static
$X;
private static function
__init() {
   echo
'B', "\n";
  
self::$X = array(1, 2);
}
}
class
A {
  public static
$Y;
  private static function
__init() {
    echo
'A', "\n";
   
self::$Y = array(3, 4);
  }
}
?>
test.php:
<?php
function __autoload($n) {
  if (
$n == 'A' || $n == 'B') require 'test2.php';
  ... do
our __init() trick ...
}
var_dump(B::$X); // shows B, then array(2) (1, 2)
var_dump(A::$Y); // shows NULL.
?>
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1
mattsch at gmail dot com
2 months ago
You can use final methods to replace class constants.  The reason for this is you cannot unit test a class constant used in another class in isolation because you cannot mock a constant.   Final methods allow you to have the same functionality as a constant while keeping your code loosely coupled.

Tight coupling example (bad to use constants):

<?php
interface FooInterface
{
}

class
Foo implements FooInterface
{
    const
BAR = 1;

    public function
__construct()
    {
    }
}

interface
BazInterface
{
    public function
getFooBar();
}

// This class cannot be unit tested in isolation because the actual class Foo must also be loaded to get the value of Foo::BAR
class Baz implements BazInterface
{
    private
$foo;

    public function
__construct(FooInterface $foo)
    {
       
$this->foo = $foo;
    }

    public function
getFooBar()
    {
        return
Foo::BAR;
    }

}

$foo = new Foo();
$baz = new Baz($foo);
$bar = $baz->getFooBar();
?>

Loose coupling example (eliminated constant usage):

<?php
interface FooInterface
{
    public function
bar();
}

class
Foo implements FooInterface
{
    public function
__construct()
    {
    }

    final public function
bar()
    {
        return
1;
    }
}

interface
BazInterface
{
    public function
getFooBar();
}

// This class can be unit tested in isolation because class Foo does not need to be loaded by mocking FooInterface and calling the final bar method.
class Baz implements BazInterface
{
    private
$foo;

    public function
__construct(FooInterface $foo)
    {
       
$this->foo = $foo;
    }

    public function
getFooBar()
    {
        return
$this->foo->bar();
    }

}

$foo = new Foo();
$baz = new Baz($foo);
$bar = $baz->getFooBar();
?>
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1
cottton at i-stats dot net
3 months ago
imo good to know:
<?php
class BaseClass
{
    protected static
$var = 'i belong to BaseClass';

    public static function
test()
    {
        echo
'<hr>'.
           
'i am `'.__METHOD__.'()` and this is my var: `'.self::$var.'`<br>';
    }
    public static function
changeVar($val)
    {
       
self::$var = $val;
        echo
'<hr>'.
           
'i am `'.__METHOD__.'()` and i just changed my $var to: `'.self::$var.'`<br>';
    }
    final public static function
dontCopyMe($val)
    {
       
self::$var = $val;
        echo
'<hr>'.
           
'i am `'.__METHOD__.'()` and i just changed my $var to: `'.self::$var.'`<br>';
    }
}

class
ChildClass extends BaseClass
{
    protected static
$var = 'i belong to ChildClass';

    public static function
test()
    {
        echo
'<hr>'.
           
'i am `'.__METHOD__.'()` and this is my var: `'.self::$var.'`<br>'.
           
'and this is my parent var: `'.parent::$var.'`';
    }
    public static function
changeVar($val)
    {
       
self::$var = $val;
        echo
'<hr>'.
           
'i am `'.__METHOD__.'()` and i just changed my $var to: `'.self::$var.'`<br>'.
           
'but the parent $var is still: `'.parent::$var.'`';
    }
    public static function
dontCopyMe($val) // Fatal error: Cannot override final method BaseClass::dontCopyMe() in ...
   
{
       
self::$var = $val;
        echo
'<hr>'.
           
'i am `'.__METHOD__.'()` and i just changed my $var to: `'.self::$var.'`<br>';
    }
}

BaseClass::test();  // i am `BaseClass::test()` and this is my var: `i belong to BaseClass`
ChildClass::test(); // i am `ChildClass::test()` and this is my var: `i belong to ChildClass`
                    // and this is my parent var: `i belong to BaseClass`
ChildClass::changeVar('something new'); // i am `ChildClass::changeVar()` and i just changed my $var to: `something new`
                                        // but the parent $var is still: `i belong to BaseClass`
BaseClass::changeVar('something different'); // i am `BaseClass::changeVar()` and i just changed my $var to: `something different`
BaseClass::dontCopyMe('a text'); // i am `BaseClass::dontCopyMe()` and i just changed my $var to: `a text`
ChildClass::dontCopyMe('a text'); // Fatal error: Cannot override final method BaseClass::dontCopyMe() in ...
?>
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-1
t at bestcodepractise dot com
6 months ago
@someone
@thomas
Decorating a finalized class is not possible. The decorator that's mentioned is incomplete. There's a fundamental flaw in it. Look:
<?php
//copy'n'paste your FooBar, Foo definition

$f = new Foo;
$fd = new FooDecorator($f);

var_dump($fd instanceof $f);  //FALSE
var_dump(is_a($fd, 'Foo')); //FALSE
fooFoo($fd);  //E_RECOVERABLE_ERROR here !!!;

?>
What you've created is just an object that happens to have the same methods(a duck type). But if in the client code someone makes decision based on the type of your passed decorator they'll make incorrect decision - or to be more precise not the one that you, the author of the 'decorator' wants them to make.
FYI that's the correct implementation based on GoF:
<?php
class Foo
{
    public
method doFoo()
    {
       
// do something useful and return a result
   
}
}

//decorator must inherit the interface(the methods, type info etc.) //of the decorated class by *extending* it.
class FooDecorator extends Foo
{
    private
$foo;
  
    public function
__construct(Foo $foo)
    {
       
$this->foo = $foo;
    }
  
    public function
doFoo()
    {
         
$result = $this->foo->doFoo();
         
// ... customize result ...
         
return $result;
    }
}

function
fooFoo(Foo $f) {}
$f = new Foo;
$fd = new FooDecorator($f);

var_dump($fd instanceof $f); //true
fooFoo($fd); //no E_RECOVERABLE_ERROR here;

?>

I haven't come across any legitimate use of a finalizing class/method and I personally think that 'final' is has no much use and is just a copy'n'pasted from Java into PHP. The keyword makes code difficult to test. If you have to create a test double from a finalized class because you'll need create a derived type to shadow the methods you don't care about. If one of them is finalized you've already lost.
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-34
thomas at somewhere dot com
8 months ago
Please, please, please never use this keyword anywhere in your code. By using it you are saying that you somehow magically know that your class or method is perfect and will suit every possible use of the class API you're defining and that no one will ever need to do so much as add some data checking before calling parent:: on the method.

This is never true and will never be true, so let the poor programmer that needs to extend your code do so.

If you want to have "final" in your code then you don't understand OOP.

Hopefully this will be removed in some future version of the language.
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