6CC001 Workshop - week 03

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Static classes - An example

Global variables undermine OO programming:

• If a class relies on a certain global variable to work, then it can not be reused in another application.
• It also causes tight coupling: 2 parts of a system tightly bound together, so that a change in one part necessitates a change in the other part.

However using some sort of global variable mechanism can be tempting, and sometimes more efficient than getting your classes to pass information all around the system. Let’s create a simple “Preferences” class, using static properties and methods.

// Preferences.php

class Preferences
{
  private static $values = array();
  
  static function set($key, $val)
  {
    self::$values[$key] = $val;
  }
  
  static function get($key)
  {
    return self::$values[$key];
  }
}

Let's test our new class:

// test.php

require_once("Preferences.php");

//Now one object could store a value...
Preferences::set("db_server", "mi-linux");

// ...

// ... that could then be retrieved by another object:
$db_server = Preferences::get("db_server");
echo $db_server;

Abstract classes - an example

An abstract class cannot be instantiated, i.e. you cannot use the “new” operator to create objects.

An abstract class defines (and optionally partially implements) the interface for any class that might extend it (via inheritance).

abstract class Shape
{
  protected $height;
  protected $width;
  
  public function __construct($height, $width)
  {
    $this->height = $height;
    $this->width = $width;
  }
  
  abstract public function CalculateSurface();
} 

This “Shape” class is telling us:

• I am abstract, and as such cannot be used directly. I am only a rough template that other classes can build on via inheritance.
• All my “children” classes will have $height and $width attributes, as well as a standard constructor method.
• They must also have a CalculateSurface method, but I leave it to them to implement how that bit works.

A first child class could be:

class Rectangle extends Shape
{
  public function CalculateSurface()
  {
    return $this->height * $this->width;
  }
}

This “Rectangle” class is telling us:

• I wish to extend the Shape class, and as such I inherits the $height and $width attributes, as well as the standard constructor method.
• Furthermore, and as per “contract”, I provide an implementation for the CalculateSurface method.

Likewise, another child class could be :

class Circle extends Shape
{
  public function __construct($height)
  {
    $this->height = $height;
    $this->width = $height;  
  }
  public function CalculateSurface()
  {
    $radius = $this->height/2;
    return 3.14 * $radius * $radius;
  }
}

This “Circle” class is telling us:

• I wish to extend the Shape class, but I choose to override the constructor method (since being a circle means my height and width are equal)
• Furthermore, and as per “contract”, I provide an implementation for the CalculateSurface method.

You can instantiate the Circle and Rectangle classes, as they are not abstract:

$rectangle = new Rectangle(50, 30);
$circle = new Circle(10);

print $rectangle->CalculateSurface()."<br>";
print $circle->CalculateSurface()."<br>";

Exceptions - an example

PHP 5 finally introduces exceptions!

• Exceptions are special objects that handle unexpected results and errors.
• The code producing the error throws an exception, and the code handling the error catches the exception.

A simple example:

class FileReader
{
  function __construct($filename)
  {
    if(!file_exists($filename))
      throw new Exception("File {$filename} doesn’t exist");
  }
}

Our “FileReader” object might detect an error (e.g. the file doesn’t exist). But it hasn’t got enough contextual information to know what to do about it. If we were to make our class more aware of its context, it would become less reusable!

So it simply throws an exception, and lets the calling code sort it out:

try
{
  $file = new FileReader('not_there.txt');
}
catch(Exception $e)
{
  die ($e->getMessage());
}