Final classes
Inheritance allows for enormous flexibility within a class hierarchy. You can create subclasses to extend the functionality of a base class, but PHP OOP gives the posibility to create classes that cannot be extended. Such a class is called
final class.
A
final class is declared by adding the
final keyword before the
class word.
- For example:
<?php
// base class (declared final)
final class Base {
// ... PHP code
}
// define a child class derivated from Base
class BaseChild extends Base {
// ... PHP instructions
}
?>
Running this code will generate the following error:
Fatal error: Class BaseChild may not inherit from final class (Base) in ...
Final methods
A normal method (public or protected) can be overridden in the child class. If you want a method to remain fixed and unchanging, prefix the definition with the
final keyword.
A final method cannot be overridden.
- For example:
<?php
// base class
class Base {
// final method
final public function testMethod() {
echo 'This is a final method';
}
}
// define a child class derivated from Base
class BaseChild extends Base {
// override the testMethod()
public function testMethod() {
echo 'Another text';
}
}
?>
This example will generate the following error:
Fatal error: Cannot override final method Base::testMethod() in ...
By declaring a class or method final, you limit the flexibility given by inheritance, so, use final with care.
Daily Test with Code Example
HTML
CSS
JavaScript
PHP-MySQL
Which HTML element can be used to embed a SWF flash content?
<object> <div> <script><object type="application/x-shockwave-flash" data="file.swf" width="500" height="250">
<param name="src" value="file.swf" />
Your browser not support SWF.
</object>
Which CSS pseudo-class adds a style to an input form field that has keyboard input focus?
:active :focus :hoverinput:focus {
background-color: #88fe88;
}Click on the instruction which converts a JSON string into a JavaScript object.
JSON.stringify(javascript_object) object.toString() JSON.parse(json_string)var jsnstr = '{"url": "http://coursesweb.net/", "title": "Web Development Courses"}';
var obj = JSON.parse(jsnstr);
alert(obj.url);Indicate the PHP function which can be used to create or write a file on server.
fopen() file_put_contents() file_get_contents()if (file_put_contents("file.txt", "content")) echo "The file was created";
else echo "The file can not be created";