Final 关键字

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PHP 5 新增了一个 final 关键字。如果父类中的方法被声明为 final，则子类无法覆盖该方法。如果一个类被声明为 final，则不能被继承。

Example #1 Final 方法示例

  <?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 类示例

  <?php
 final class  BaseClass  {
   public function  test () {
       echo  "BaseClass::test() called\n" ;
   }
   
    // 这里无论你是否将方法声明为final，都没有关系
    final public function  moreTesting () {
       echo  "BaseClass::moreTesting() called\n" ;
   }
}

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

Note: 属性不能被定义为 final，只有类和方法才能被定义为 final。

用户评论:

[#1] xavier dot inghels at gmail dot com [2015-03-31 09:47:03]

Final is not really about overriding or overloading a method.
PHP doesn't support (yet ?) method ovverriding.

The final keyword however prevent you to redefine a method already previously declared.

[#2] mattsch at gmail dot com [2014-06-03 19:24:56]

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(); ?>

[#3] cottton at i-stats dot net [2014-05-12 18:02:59]

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 ... ?>

[#4] t at bestcodepractise dot com [2014-02-22 22:18:51]

@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.

[#5] someone dot else at elsewhere dot net [2014-01-04 22:40:54]

@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; } } ?>

[#6] thomas at somewhere dot com [2013-12-07 22:25:20]

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.

[#7] Anonymous [2010-09-28 07:06:20]

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.

[#8] Baldurien [2010-08-31 13:37:50]

"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. ?>

[#9] jriddy at gmail dot com [2009-07-17 14:20:16]

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

[#10] santoshjoshi2003 at yahoo dot co dot in [2008-12-05 02:45:58]

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

[#11] slorenzo at clug dot org dot ve [2007-10-30 23:13:48]

<?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 ?>

[#12] penartur at yandex dot ru [2007-03-22 01:39:07]

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.