Object Oriented Programming¶

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

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.

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.

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.

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.

PHP example¶

$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!';
    }
}

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();
}

Python Example¶

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!