Introduction to Object-Oriented Programming

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Introduction to Object-Oriented Programming

• Sandy GrahamUniversity of Waterloofor
  theImperial Oil Summer Institutefor Computer
  Studies Teachers
• August 2003

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What is O-O?

• Design, programming, and languages
• An alternative to procedural programming
• anything done using O-O can also be done using a
  procedural paradigm
• Considering a problem from the perspective of
  objects and how they interact

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Skills for O-O Programming

• Must have a solid understanding of subprograms
  and parameters
• stepwise refinement
• Should understand references/pointers
• Coding still involves basic concepts
• selection structures, repetition structures,

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O-O vs. Procedural

• 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

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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
  existing class

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Objects

• Part of a program which
• models some real or conceptual object
• has behavioural responsibilities (behaviours)
• has informational responsibilities (attributes)

• Behaviours (methods)
• things an object can do
• like procedures and functions in other languages
• Attributes (fields)
• information an object knows (has-a)
• like data and variables in other languages
  (records)

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Classes

• A class is a template. No data is allocated until
  an object is created from the class. However
  attributes and behaviours are generically defined
• The creation (construction) of an object is
  called instantiation. The created object is often
  called an instance (or an instance of class X)

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Analogies for Classes Objects

• Class
• blueprint
• pattern
• cookie cutter
• factory

• Object
• building
• garment
• cookie
• widget

Platos allegory of the cave.
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Class Diagram
Internal/private/helper methods only available
within the class.
Defined by the class. Filled by the object.
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Encapsulation - Real Life

• Bank machine
• Hidden data
• account balance
• personal information
• Interface
• deposit, withdraw, transfer
• display account information

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Multiple Instances of a Class

• No limit to the number of objects that can be
  created from a class
• Each object is independent. Changing one object
  doesnt change the others

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Interaction Among Classes

• A program is composed of multiple classes
• Classes may contain references to other classes
  within the set of attributes or behaviours
• Start in an application class (main)
• construct one or more objects and call methods
  associated with those objects

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Using UML in Design
Bank

• every class has a box
• class name
• attributes/instance variables
• behaviours/methods
• Arrows indicate the relationships among classes
• indicates 0-many

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Inheritance

• The inheriting class contains all the attributes
  and behaviours of the class it inherited from
  plus any attributes and behaviours 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

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Inheritance Diagram
General Outline
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Why Use Inheritance?

• Modular coding
• less code, easier to understand
• Code reuse
• dont break what is already working
• easier updates
• May not have access to modify the original source
  code
• Polymorphism

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Inheritance Terminology

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

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Inheritance Bank Account
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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
  existing class