archived-doc-en/language/oop5/final.xml

<?xml version="1.0" encoding="utf-8"?>
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<sect1 xml:id="language.oop5.final" xmlns="http://docbook.org/ns/docbook">
 <title>Final Keyword</title>
 <para>
  The final keyword prevents child classes from overriding a method, property, or constant by
  prefixing the definition with <literal>final</literal>. If the class
  itself is being defined final then it cannot be extended.
 </para>
 <para>
  <example>
   <title>Final methods example</title>
   <programlisting role="php">
<![CDATA[
<?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()
?> 
]]>
   </programlisting>
  </example>
 </para>
 <para>
  <example>
   <title>Final class example</title>
   <programlisting role="php">
<![CDATA[
<?php
final class BaseClass {
   public function test() {
       echo "BaseClass::test() called\n";
   }

   // As the class is already final, the final keyword is redundant
   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)
?> 
]]>
   </programlisting>
  </example>
 </para>
 <example>
  <title>Final property example as of PHP 8.4.0</title>
  <programlisting role="php">
<![CDATA[
<?php
class BaseClass {
   final protected string $test;
}

class ChildClass extends BaseClass {
    public string $test;
}
// Results in Fatal error: Cannot override final property BaseClass::$test
?>
]]>
  </programlisting>
 </example>
 <example xml:id="language.oop5.final.example.php81">
  <title>Final constants example as of PHP 8.1.0</title>
  <programlisting role="php">
<![CDATA[
<?php
class Foo
{
    final public const X = "foo";
}

class Bar extends Foo
{
    public const X = "bar";
}

// Fatal error: Bar::X cannot override final constant Foo::X
?>
]]>
  </programlisting>
 </example>

 <note>
  <simpara>
   As of PHP 8.0.0, private methods may not be declared final except for the <link linkend="language.oop5.decon.constructor">constructor</link>.
  </simpara>
 </note>
 <note>
  <simpara>
   A property that is declared <link linkend="language.oop5.visibility-members-aviz"><literal>private(set)</literal></link> is implicitly <literal>final</literal>.
  </simpara>
 </note>
</sect1>
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