Object Oriented Programming

1
Object Oriented Programming

• A one hour introduction to O-O.
• Originally written by Tom West, Holt Software
• Edited by Jaye Herbert, Sir Frederick Banting S.S.

2
Grade 12 Expectations

• Specific
• Describe the difference between procedural and
  object-oriented programming
• Describe how procedural and object-oriented
  programming paradigms can be used to solve
  different problems

3
Grade 12 Expectations

• Explain the importance of designing reusable code
  for large software projects
• Incorporate appropriate code from shared software
  libraries into software projects
• Build and maintain a small software library to
  facilitate the reuse of code
• Develop software libraries in project settings
• Use predefined modules from software libraries to
  improve productivity

4
Why O-O?

• Programs are getting too large to be
  comprehensible in-total by any human being
• CS desperately looking for a way of cognitively
  managing super-large projects
• Object Oriented paradigm allows programmers to
  use large blocks of code with a much smaller
  cognitive load
• Makes code re-use a real possibility

5
Why O-O?

• Benefits only occur in larger programs
• Analogous to structured programming
• Progs lt 30 lines, spaghetti is as understandable
  and faster to write than structured
• Progs gt 1000 lines, spaghetti is
  incomprehensible, probably doesnt work, not
  maintainable
• O-O greatest benefits are seen for programs of gt
  1000 lines

6
Why O-O?

• Much of the benefit is in easier maintenance of
  large projects
• Example of O-O strength
• Write a library automation system handling books
  and periodicals.
• Now you are given one week to modify program to
  handle video tapes, audio tapes and software!

7
What is O-O?

• Cannot do anything using OO that cant be
• done using procedural paradigm
• Procedural Paradigm
• Program defines data and then calls subprogram to
  act on the data
• Object Paradigm
• Program creates objects that encapsulate the data
  and procedures that operate on the data

8
OOP Concepts

• A program models a world of interacting objects
• Objects create other objects and send messages
  to each other (in Java, call each others
  methods)
• Each object belongs to a class a class defines
  properties of its objects
• A class implements an ADT the data type of an
  object is its class
• Programmers write classes (and reuse existing
  classes)

9
Procedural vs. O-O
Procedural

Object 1

Object Oriented

Data

Object 2

Main

Program
Data


Object 3

Data

10
Basic Concepts

• Objects
• Hidden data that is accessed and manipulated
  through a well defined interface
• Classes
• A template or "blueprint" for creating objects,
  each with their own data
• Inheritance
• A new class created by modification of an already
  existing class

11
ObjectsLong Definition

• Code containing a set of data, all the routines
  to manipulate the data, and an interface
• The set of data are often called the fields of
  the object
• The routines to manipulate the data are often
  called the methods of the object
• The interface of an object is simply the subset
  of methods that other programs are allowed to
  call. The names and parameters of the methods in
  the interface should only rarely change

12
ObjectsDiagram
General Outline
Interface
imports Object Rest of the Program
method
method
Fields of The Object
method
method
method
method
method
PhoneBook Example

PhoneBook



imports PhoneBook PhoneBook Demo Program
Add
numEntries book (array of records

numEntries




Delete



book
(array


Find

of records)

LookUp

13
Objects Why Use Them? (1)

• To use an object, the user need only comprehend
  the interface. No knowledge of the internals are
  necessary
• Self-contained. Once the interface is defined,
  the programmer can implement the interface (write
  the object) without interference of others

14
Objects Why Use Them? (2)

• Implementation can change at a later date without
  rewriting any other part of the program (as long
  as the interface doesn't change)
• Changes in the data mean changing code in one
  location, rather than code scattered around the
  program (where it is easy to miss)

15
ObjectsProgramming

• Hidden data that is accessed and manipulated
  through a well defined interface
• PhoneBook
• Hidden Data Set of names and phone s
• Number of names in book
• Interface add, delete, change, lookUp

16
Objects Interface

• Why Hide the Data?
• Allows implementer free hand to write and change
  the object
• PhoneBook object
• Allows user to enter, look up and delete names
  and phone numbers
• Implemented using an array
• Maximum 100 names in the phone book

17
ObjectsInterface Example

• Example
• The PhoneBook object is successful. It is used in
  hundreds of applications across the company
• It only holds 100 records! It now must upgraded
  to hold unlimited number of records
• How do we do so without breaking all the other
  programs in the company?

18
ObjectsInterface Example cont

• The interface does not need to change. Thus there
  is no need to change any of the programs using
  PhoneBook object
• If this had been programmed in the procedural
  paradigm, each program that used the phone book
  would have had a copy of the data array and would
  have to have been extensively modified to be
  upgraded

19
ObjectsInterface Example (4)

• If we had given access to the fields of the
  object, other programs might have made use of the
  names, phones or numEntries fields and would
  break with new version of PhoneBook

20
ClassesLong Definition

• Code looks superficially similar to a class
  ("static" is removed)
• A class is a template. No data is allocated until
  an object is created from the class
• The creation of an object is called
  instantiation. The created object is often called
  an instance (or an instance of class x)
• No limit to the number of objects that can be
  created from a class
• Each object is independent. Changing one object
  doesn't change the others

21
ClassesDiagram
Object 1
Fields for 1
Object 2
Main Program
Fields for 2
Object 3
Fields for 3
22
ClassesWhy Use Them?

• A template or "blueprint" for creating objects,
  each with its own data
• Without classes, each otherwise identical object
  must have its own object, differing only in name
• Objects created (instantiated) from a class can
  be assigned to variables and manipulated as such.
  For example, a program can have arrays of objects

23
ClassesWhy Use Them?

• In general, use classes instead of objects. Any
  number of objects can be created from a class and
  as variables, the objects created from a class
  are more flexible
• Example
• It may well turn out that one of the programmers
  wants to use two PhoneBook objects in the same
  program (separate PhoneBooks for professional and
  personal contacts). If we wrote it as a class,
  then no extra work is required on our part

24
ClassesProgramming

• A template or "blueprint" for creating objects,
  each with their own data PhoneBook
• Hidden Data
• Set of names and phone s
• Number of names in book
• Interface
• Add, Delete, Change, LookUp

25
Encapsulation

• Means that all data members (fields) of a class
  are declared private. Some methods may be
  private, too.
• The class interacts with other classes (called
  the clients of this class) only through the
  classs constructors and public methods.
  Constructors and public methods of a class serve
  as the interface to classs clients.

26
Inheritance

• A new class created by modification of an already
  existing class
• The inheriting class contains all the methods and
  fields of the class it inherited from plus any
  methods and fields it defines
• The inheriting class can override the definition
  of existing methods by providing its own
  implementation
• The code of the inheriting class consists only of
  the changes and additions to the base class

27
Inheritance Why Use Them? (1)

• Frequently, a class is merely a modification of
  another class. In this way, there is minimal
  repetition of the same code
• Fixing a bug in the base class automatically
  fixes it in the subclasses
• Adding functionality in the base class
  automatically adds it in the subclasses

28
Inheritance Why Use Them? (2)

• Less code makes the program easier to understand
• There is less chance of different (and
  inconsistent) implementations of the same
  operation

29
InheritanceTerminology

• Class one above
• Parent class
• Class one below
• Child class
• Class one or more above
• Ancestor class, Base class
• Class one or more below
• Descendent class

30
Basic Concepts

• Objects
• Hidden data that is accessed and manipulated
  through a well defined interface
• Classes
• A template or "blueprint" for creating objects,
  each with their own data
• Inheritance
• A new class created by modification of an already
  existing class

31
Terms to Lookup

• Overriding
• Overloading
• Polymorphism
• super
• this
• clone
• constructor