Objectoriented concepts

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Object-oriented concepts UML 1

• LectureConcepts and ApproachesBasis of the
  Object-Oriented Paradigm

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An Introduction to Objects

• Basis of the Object-Oriented Paradigm

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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 Dictionary
• A 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 Computersetc. etc. etc. etc.
  etc. etc. etc. ...

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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
  object
• An object can manipulate (change, copy, delete)
  its data

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Objects Display Behaviour

• Behaviour is how an object acts and reacts, in
  terms of its state changes and message passing
• Think of behaviour as 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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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 is never operated
  uponServers - Only ever operated
  uponAgents - Both operates on and is operated
  upon

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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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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
• Lets look at an exampleMammal describes a class
  of animals which possess the attributes and
  behaviours associated with being a mammal (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
• Implementation of a class is its internal view,
  ie. what states it possess and which behaviours
  it exhibits.

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Class Interface Visibility

• Public Accessible to all clientsProtected acces
  sible to itself, subclasses and its
  friendsPrivate accessible only to itself
  and its friendsIn other words
• well formed objects must only allow friends
  to access their protected and/or private parts
  J

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Class Relationships

• Associations
• Inheritance
• Aggregation
• Utilisation
• Instantiation

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Class Associations

• Denotes a semantic dependency between classes of
  objects
• Classes possess roles in their relationships
• Class associations have cardinality (one-to-one,
  one-to-many, many-to-many, n-m)

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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
A
D
B
B inherits directly from A
E
C
C inherits directly from B, therefore also
inheriting indirectly from A
E inherits directly from both A and D
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Class Aggregations

• Similar to the aggregation exhibited by objects
• Whole/Part relationship (Container/Component)
• Physical containment and containment by reference
  is possible

ShareHolder
ShareHolder is not actually constructed with
Shares but still contains them
Shares
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Class Utilisations

• One class acts as the Server (or Supplier) to
  another Client class

Server/Client
Server
Uses
Uses
Client
Uses
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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