An Introduction to Object Orientation

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An Introduction to Object Orientation

• Introduction to Objects
• The Object-Oriented Approach
• An introduction to OO analysis, design and
  programming
• OO vs Frames

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What are Objects?

• A thing presented or capable of being presented
  to the senses a thing observed .. that upon
  which attention, interest or some emotion is
  fixed Chambers DictionaryA tangible and/or
  visible thing, something that may be apprehended
  intellectually, something towards which thought
  or action is directedBooch

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quite, but What are Objects?

• Examples (of which there are many)PeopleMotor
  CarsPersonal Computersbut also Bank Accounts
  and addresses

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Objects Possess State

• The state of an object encompasses all of the
  (usually static) properties of the object plus
  the current (usually dynamic) values of each of
  these properties
• Think of state as the data possessed by an
  objectAn object can manipulate (change, copy,
  delete) its data

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Objects Display Behaviour

• Behaviour is how an object acts and reacts, i.e.
  an objects internal functions
• Objects are responsible for instructing their own
  functions to operate and for requesting other
  objects to instruct their own functions

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Objects Have Identity

• Identity is that property of an object which
  distinguishes it from all other objects
• Think of identity as being represented by an
  objects name
• In the O-O model, all objects have completely
  unique names

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Example - a watch

• Identity - My watch
• Sate (data) time
• Behavior
• Displays current time
• Set time
• Stop watch
• Start watch

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Objects Communicate

• Objects communicate with one another
• This is achieved by message passing

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Objects Have Relationships

• LinksA physical or conceptual connection
  between objectsActors - Operates on other
  objects but are never operated uponServers - Only
  ever operated uponAgents - Both operate on and
  are operated upon

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Objects Belong to Classes

• Class and Object are tightly interwoven
• A class is defined as a group, set or kind
  marked by common attributes or a common
  attribute a group division, distinction or
  rating based on quality, degree of competence or
  conditionWebsters Dictionary

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ok, so What is a Class?

• In other words, a class is a set of objects that
  share a common structure and a common behaviour
• A single object is simply an instance of a class
  e.g.
• Mammals a class of animals which possess common
  attributes and behaviour (eg. Cows, Monkeys,
  Humans etc) such as hair, suckle young,
  warm-blooded, sexual, smelly etc.

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Class Interfaces and Implementation

• Interfaces allow classes to provide a view of
  their internal structure and behaviour to the
  outside world. Usually this is restricted to hide
  none relevant details, e.g. a watch.
• Implementation of a class is its internal view,
  ie. what states it possess and which behaviours
  it exhibits.

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Interface Visibility

• Public Accessible to all clients
• Protected accessible to itself, subclasses
  and its friends
• Private accessible only to itself and its
  friends

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Objects Have Relationships

• AggregationDenotes a whole/part hierarchy, for
  example

Airplane
Container
Cockpit
Wings
Container/Component
Seats
Engines
Container/Component
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Class Relationships

• Inheritance
• Aggregation
• Instantiation
• and also
• Associations
• Utilisation

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Class Inheritance

• A class can inherit both attributes and behaviour
  from one or more superclasses (a parent/child
  relationship)Inherited characteristics can be
  altered for that particular class
  (Polymorphism)Inheritance is hierarchical

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Class Inheritance
Living Entity
Food source
Animal
Plant
Animal inherits directly from Living entity
Mammal
Plant inherits directly from both Living entity
and Food source
Mammal inherits directly from Animal, therefore
also inheriting indirectly from Living entity
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Class Aggregations

• Similar to the aggregation exhibited by objects
• Whole/Part relationship (Container/Component)

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Class Instantiations

• An actual object in the O-O model is instantiated
  from a single class
• Thus every object is the instance of some class,
  and each class can have zero or more object
  instances
• Hence, classes are static, object instances are
  dynamic

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A Class Example - Grand Prix Drivers
Mother
Father
Driving Skill (Inheritance)
Racing Car
Drives (Association)
Contains (Aggregation)
Person
Birth (Instantiation)
Powerful Engine
Pit Stops (Utilisation)
Damen Hill
Pit Crew
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The OO Approach

• Adopting the Object Paradigm

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Categories of Analysis and Design

• Structured Analysis and Design (eg.
  SSADM)Data-Oriented Analysis and Design (eg.
  JSP)Object-Oriented Analysis and Design (see
  later)

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The Object Model

• A conceptual framework, a specific way of
  thinkingThree stages in using O-O to model the
  world-OOA (Object-Oriented Analysis)OOD
  (Object-Oriented Design)OOP (Object-Oriented
  Programming)

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Object-Oriented Analysis

• Examines user requirements
• Produces a model of the problem
• Focus on classes rather than actual object
  instances
• Initial stage prior to design
• Analysis model feeds into design process

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Object-Oriented Design

• Provides solutions to the problem modelled during
  analysis
• Follows on from the analysis process and feeds
  into the implementation stage
• Uses notations to express the logical and
  physical models of the system
• Concentrates on depicting both static and dynamic
  models

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Object-Oriented Programming

• Implements the design models as actual computer
  code
• Utilises some or all of the principles employed
  by object-orientation
• O-O languages include C, Eiffel, Smalltalk,
  Object Pascal, Simula

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Object Model Elements

• Abstraction
• Encapsulation
• Modularity
• Hierarchy

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Abstraction

• Denotes the essential characteristics of an
  object
• Crisply defined conceptual boundaries
• Relative to the perspective of the viewer
• Abstract classes and objects may not actually
  exist

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Encapsulation

• Aka Information hiding
• Abstraction and Encapsulation are complementary
• Compartmentalises the state and behaviour of
  classes and objects that need to be hidden from
  other classes and objects

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Modularity

• Partitioning into individual components to help
  reduce complexity
• Achieved by the use of objects and classes
• We require high cohesion (objects and classes)
  and loose coupling (relationships and message
  passing)

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Hierarchy

• The ranking or ordering of abstractions
• Inheritance is hierarchical
• Subclasses inherit from their superclasses
  (generalisation/specialisation relationship)
• Other relationships can be hierarchical, eg.
  aggregation

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Digging out Objects, Classes and Attributes

• Look for Nouns, these provide classes, attributes
  and object instances (proper nouns)The cat sat
  on the mat. Cat and Mat are classesA cat called
  Tiddles sat on a mat called Martin. Tiddles is an
  instance of class Cat, Martin is an instance of
  class MatA cat with 10 whiskers sat on a mat of
  length 2m. Whiskers is an attribute of class Cat
  and has value 10, Length is an attribute of class
  Mat and has value 2m

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Digging out Relationships and Behaviours

• Look for Verbs, these provide insight into the
  relationships between classes/objects and the
  behaviours of classes/objectsThe cat sat on the
  mat. Class Cat has an association relationship
  with class Mat by sitting on it.The cat, which
  is made up from a cat body, cat head, 4 cat limbs
  and a cat tail, sat on the mat. Class cat has a
  11 aggregation relationship with classes Cat
  Body, Cat Head, Cat Tail and a 14 aggregation
  relationship with class Cat Limbs

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Digging out Relationships and Behaviours

• The purring cat sat on the flying mat. To purr
  is a behaviour of the class Cat To fly is a
  behaviour of the class matThe tiger, which is a
  member of the cat family, sat on the rug, which
  is a kind of mat . Class Tiger inherits from the
  Cat class Class Rug inherits from the Mat class

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OO vs Frames

• OO and frames both support
• Abstraction
• Hierarchy
• Inheritance,
• Methods or Demons
• Aion DS supports both knowledge bases in the form
  of rules and OO programming

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Summary

• In this lecture we have looked at
• Introduction to Objects
• The Object-Oriented Approach
• An introduction to OO analysis, design and
  programming
• OO vs Frames