CS 200 - Object Oriented Programming

What is depicted above?

• A mammal has fur and nurses its young.
• A dog ISA mammal and has four legs and a tail. You may infer that a dog has fur and nurses its young.
• A poodle ISA dog. You may infer that it has four legs and a tail. However, you may override defaults, e.g., a Mexican hairless dog or an amputee pooch.

Terminology

Class:

A user-defined prototype for an object that defines a set of attributes that characterize any object of the class. The attributes are data members (class variables and instance variables) and methods, accessed via dot notation.

class course: 

Class variable:

A variable that is shared by all instances of a class. Class variables are defined within a class but outside any of the class's methods. Class variables are not used as frequently as instance variables are.

count = 0
classes = []

Data member:

A class variable or instance variable that holds data associated with a class and its objects.

count = 0
self.count = self.__class__.count
or
self.count = course.count

Function overloading:

The assignment of more than one behavior to a particular function. The operation performed varies by the types of objects or arguments involved.

def __init__(self, title, dept):

Instance variable:

A variable that is defined inside a method and belongs only to the current instance of a class.

self.title = title

Inheritance:

The transfer of the characteristics of a class to other classes that are derived from it.

class gradcourse(course):

    def __init__(self, title, dept):
        course.__init__(self, title, dept)
        self.grad = True

Instance:

An individual object of a certain class. An object obj that belongs to a class course, for example, is an instance of the class course.

c = course("CS200", "Computer Science")

Instantiation:

The creation of an instance of a class.

c = course("CS200", "Computer Science")

Method:

A special kind of function that is defined in a class definition.

def add_student(self, item):

Object:

A unique instance of a data structure that's defined by its class. An object comprises both data members (class variables and instance variables) and methods.

c = course("CS200", "Computer Science")

Operator overloading:

The assignment of more than one function to a particular operator.

a = 1 + 2
b = "hello " + "world"

Examples

The Big Picture

The most common reasons to use Object Oriented Programming:

Code reuse

This one's easy (and is the main reason for using OOP). By supporting inheritance, classes allow you to program by customization instead of starting each program from scratch.

Encapsulation

Wrapping up implementation details behind object oriented interfaces insulates users of a class from code changes.

Structure

Classes provide new local scopes, which minimizes name clashes. They also provide a natural place to write and look for implementation code, and to manage object state.

Maintenance

Classes naturally promote code factoring, which allows us to minimize redundancy. Thanks both to the structure and code reuse support of classes, usually only one copy of the code needs to be changed.

Consistency

Classes and inheritance allow you to implement common interfaces, and hence create a common look and feel in your code; this eases debugging, comprehension, and maintenance.

Polymorphism

This is more a property of OOP than a reason for using it, but by supporting code generality, polymorphism makes code more flexible and widely applicable, and hence more reusable.

Other

And, of course, the number one reason students gave for using OOP: it looks good on a resume! (OK, I threw this one in as a joke, but it is important to be familiar with OOP if you plan to work in the software field today.)

Finally, ... you won't fully appreciate OOP until you've used it for a while. Pick a project, study larger examples, work through exercises -- do whatever it takes to get your feet wet with OO code; it's worth the effort.

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