Appion

h1. Object Oriented Programming

This page provides details of the major features of object oriented programming and definitions of terminology.

h2. Objects

* An _object_ often models a real-world thing - person, animal, shape, car 

* Collection of _attributes_ and _behaviors_

* Defined as a _class_ with _member variables_ and _methods_

* An _instance_ of a class is an occurance of an object that has been created using the class _constructor_ 

* A class _constructor_ method is called automatically when a new instance of a class is created. 

* A _destructor_ method is called automatically when an instance of a class is destroyed.

h2. Encapsulation

* Bundle data (attributes) with the methods that operate on the data

* Restrict access to the data using _access modifiers_ like _private_ (only accessable within self) and _protected_ (available to subclasses as well)

* Best Practices 

** Keep member variables private

** Provide accessor functions to attributes when needed

** Public methods provide an interface to the rest of the world, everthing else should be private

* Benefit: reduce complexity, increases robustness, by limiting the interdependencies between components.

h2. Inheritance

* Reuse code by basing an object on another object

* Terminology

** Superclass, base class, parent class 

** Subclass, derived class, child class

* Examples

** parent: Animal, children: Person, Cat, Fish

** parent: MotorVehicle, children: Car, Truck, Bus, Tractor

** parent: Shape, children: Ellipse, Rectangle, Triangle, Cone

* _Child classes_ inherit attributes and behaviors from _parent classes_

* Child classes may _override_ behaviors inherited from parent classes by providing it's own implementation of a method.

* An _abstract method_ in a parent class does not have an implementation. Child classes MUST provide an implementation to be instantiated. 

* An _abstract class_ has at least one abstract method and cannot be instantiated.

* A _virtual method_ in a parent class provides a default implementation that MAY be overriden by the child classes.

h2. Polymorphism

* Objects of different types (or classes) can be defined with the same interface and respond to method calls with the appropriate type-specific behavior.

* The exact type of the object is determined at run-time, so the program does not need to determine type in advance.

* Examples 

** Draw every shape in a collection of shapes. The collection has Rectangles and Triangles, both based on the Shape class, which has an abstract method called Draw().

** Make a collection of different animals talk. Code example shown below.

h2. PHP example

<pre>

$animals = array('Lassie'=>'dog', 'Mr. Mistoffelees'=>'cat', 'Missy'=>'cat' );

foreach( $animals as $name=>$type ) {

    if ( $type == 'dog' ) {

        echo $name.': Woof!';

    } else if ( $type == 'cat' ) {

        echo $name.': Meow!';

    }

}

</pre>

<pre>

interface IAnimal {

    function getName();

    function talk();

}

abstract class AnimalBase implements IAnimal {

    protected $name;

    public function __construct($name) {

        $this->name = $name;

    }

    public function getName() {

        return $this->name;

    }

}

class Cat extends AnimalBase {

    public function talk() {

        return 'Meowww!';

    }

}

class Dog extends AnimalBase {

    public function talk() {

        return 'Woof! Woof!';

    }

}

$animals = array(

    new Cat('Missy'),

    new Cat('Mr. Mistoffelees'),

    new Dog('Lassie')

);

foreach ($animals as $animal) {

    echo $animal->getName() . ': ' . $animal->talk();

}

</pre>

h2. Python Example

<pre>

class Animal:

    def __init__(self, name):    # Constructor of the class

        self.name = name

    def talk(self):              # Abstract method, defined by convention only

        raise NotImplementedError("Subclass must implement abstract method")

class Cat(Animal):

    def talk(self):

        return 'Meow!'

class Dog(Animal):

    def talk(self):

        return 'Woof! Woof!'

animals = [Cat('Missy'),

           Cat('Mr. Mistoffelees'),

           Dog('Lassie')]

for animal in animals:

    print animal.name + ': ' + animal.talk()

# prints the following:

#

# Missy: Meow!

# Mr. Mistoffelees: Meow!

# Lassie: Woof! Woof!

</pre>