OOP: Abstract classes and Interfaces

[DexterMorgan]

Hi, I have been trying to improve my object oriented programming so i have been reading various tutorials like:
http://net.tutsplus.com/tutorials/php/understanding-and-applying-polymorphism-in-php/
And have picked up a copy of PHP objects, patterns and practice

I Have read the abstract classes and interfaces section a few times but i still do not see the point in them, can anyone please explain to me why people use these features?

I understand that an interface might be good for ensuring that a class uses the required methods but I have no idea about how to implement abstract classes.
Thanks in advance.

[NogDog]

You can sort of think of an abstract class as a class that has a built-in interface, which would be any methods defined as "abstract" within it. The main functional difference compared to defining it a a class that implements a separate interface is that an abstract class cannot be instantiated: only any non-abstract children can be.

<?php

abstract class Gun
{
   abstract public function load();

   public function shoot()
   {
      echo "Squeeze the trigger.";
   }
}

class Musket extends Gun
{
   public function load()
   {
      echo "Pour in powder, ram home wadding and bullet, then insert cap.";
   }
}

class AssaultRifle extends Gun
{
   public function load()
   {
      echo "Remove empty magazine, insert full magazine.";
   }
}

$test = new Musket();
$test->load();
echo "<br />";
$test->shoot();
echo "<br />";
$test = new AssaultRifle();
$test->load();
echo "<br />";
$test->shoot(); // fatal error

Output:

Pour in powder, ram home wadding and bullet, then insert cap.
Squeeze the trigger.
Remove empty magazine, insert full magazine.
Squeeze the trigger.
Fatal error: Cannot instantiate abstract class Gun in C:\wamp\www\test\test.php on line 38

[DexterMorgan]

thanks for the reply, do you have any information about interfaces?
From what i understand you can declare methods in abstract classes, like the shoot method above, but with interfaces you can only show public function shoot(); and none of the code inside the method.

[NogDog]

Let's play with the the previous (if trivial) example, this time using an interface:

<?php
abstract class Weapon {
   protected $name;
   public function __construct($name)
   {
      $this->name = $name;
   }
}

interface RangedWeapon
{
   public function load();
   public function shoot();
}

class Bow extends Weapon implements RangedWeapon
{
   public function load()
   {
      echo $this->name . ": Take arrow from quiver and nock it on bowstring";
   }
   public function shoot()
   {
      echo $this->name . ": Pull string back, aim, and release";
   }
}

class AssaultRifle extends Weapon implements RangedWeapon
{
   public function load()
   {
      echo $this->name . ": Remove empty magazine, insert full magazine.";
   }
   public function shoot()
   {
      echo $this->name . ": Aim, squeeze trigger";
   }
}

$test = new Bow('longbow');
$test->load();
echo "<br />";
$test->shoot();
echo "<br />";
$test = new AssaultRifle('AK47');
$test->load();
echo "<br />";
$test->shoot();

Output:

longbow: Take arrow from quiver and nock it on bowstring
longbow: Pull string back, aim, and release
AK47: Remove empty magazine, insert full magazine.
AK47: Aim, squeeze trigger

Note that any place you might use type hinting when passing one of our objects, a Bow object, for example, would qualify as "Bow", "RangedWeapon", and "Weapon", as it qualifies as itself, any class it inherits, and any interface it implements or inherits.