What is Object Orientation

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What is Object Orientation?

• Lecture 1

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Thoughts

• Like modern physicists, Buddhists see all
  objects as processes in a universe, and deny the
  existence of any material substance.
• Fritjof Capra, physicist and philosopher
• What makes you, and what are your boundaries?
• Douglas Hofstadter, author of Godel, Escher, Bach

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Object Orientation

• The world is not static, but a dynamic process
• Objects that comprise the world are also dynamic
  and in constant flux
• Models of the world should take this into account
• OO models look at systems as dynamic, not static

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Advantages of OO

• Models are stable
• As entities change, the model adapts
• Construction of systems is iterative
• Stresses reuse of components
• Supported by weak coupling
• Eases development and quality management

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Five Basic Concepts

• Objects
• Messages
• Classes
• Inheritance
• Polymorphism

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Concept 1 Objects

• Definition an atomic entity formed by the union
  of state and behavior
• A bird sings
• A window shuts
• A cup runneth over
• Objects are typically nouns, behavior is a verb

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Objects

• The fundamental characteristics of an object are
• OBJECT State Identity Behavior

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Object State

• Groups values of the attributes of an object at a
  given moment in time
• State changes over time
• Attributes may be constant or variable

• CAR
• Silver- 1300 lbs- 6 gallons gas- V8 engine

• CAR
• Silver- 1292 lbs- 2 gallons gas- V8 engine

t1
t2
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Object Identity

• A characteristic that distinguishes objects from
  other similar objects
• Must be something to differentiate objects, even
  if all attributes are identical
• natural identifier
• Social security number
• Others?

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Object Behavior

• Groups the abilities (actions and reactions) of
  an object
• Behavioral components known as operations
• Object operations triggered by messages received
  from other objects

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Object Behavior
Object B
message
Object A
Triggers either operation 1 or 2 depending on
message
Operation 2
Operation 1
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Object Communication

• OO systems are collaboration of objects
• Objects work together to implement the
  functionality of desired system
• Type of communication is, therefore, important
• Categories of behavior (based on direction of
  message
• Client
• Server
• Agent

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Clients

• Objects that instigate an interaction
• Known as active objects
• Responsible for handing control over to another
  object

Agent
Client
Server
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Servers
Agent
Client

• Never instigate an interaction
• Known as passive objects
• Wait for another object to require their service
• Messaging object hands over control and then
  retakes it after service is complete

Server
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Agents
Server 1
Agent
Server 2
Client

• Combination of client and server characteristics
• Can interact with any other object
• Interactions can be result of trigger or on their
  own (like people)
• Act as filters for other objects

Server 3
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Concept 2 Messages

• Definition the unit of inter-object
  communication
• Dynamically link objects together
• Represented as

Message A
Object B
Object A
Or, if data is involved
Message A
Data B
Object B
Object A
Data A
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Categories of Messages

• Constructors
• Create objects
• Destructors
• Destroy objects
• Selectors
• Return the state of an object
• Modifiers
• Change the state of an object
• Iterators
• Visit the state of an object

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Message Synchronization

• Specifies the nature of the communication between
  objects and the rules for passing messages
• Message Broadcast Categories
• Simple
• Synchronous
• Rendezvous
• Timed
• Asynchronous

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Broadcast Categories

• Simple
• Only one object active at a time
• Sent from active to passive object
• Synchronous
• Operation triggered only when target accepts
  message
• Sender waits until acceptance

Object B
Object A
x
Object B
Object A
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Broadcast Categories
Object B
Object A

• Rendezvous
• Inverse of Synchronous
• Message sent if and only if recipient is in
  waiting for message mode
• Timed
• Sender blocked/waits for limited time for
  recipient to respond

call
Carrie
Christine
3 rings
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Broadcast Categories

• Asynchronous
• Sender is not blocked
• Sender does not wait for acknowledgement from
  recipient

Letter via US postal service
Carrie
Christine
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Representing the Object Interaction
3Z
Chan Roeun
1X
Joe
2Y
Mike
This is known as a Collaboration Diagram
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What if there are many object interactions?
8M8
Chan Roeun
3M3 5M5
1M1 4M4 10M10
9M9 7M7
Mike
2M2
6M6
Joe
Collaboration Diagram is too cluttered and
confusing
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Sequence Diagram
Chan Roeun
Joe
Mike
M1
M2
M3
M4
M5
M6
M7
M8
M9
M10
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Concept 3 Classes

• Definition of the word class
• Oxford a number of individuals having a
  character or feature in common
• Webster a number of persons or things regarded
  as forming a group by reason of common
  attributes, characteristics, qualities or traits

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Classes in everyday life

• Based on similar values for a particular, or
  group of attributes
• All purple dinosaurs all green apples all
  Jesuits
• Based on the range of an attribute
• Everyone who is between 18-21 years of age
• Based on similar behavior
• Everyone who sleeps in class everyone in the
  ECIS program

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Difficult to verbalize the rule

• Children may have their own, incomprehensible
  grouping rule
• Adults may have their own incomprehensible
  grouping rule
• Designers inability to understand users grouping
  rule may be result of not understanding business

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All about Classes

• Based on the notion of abstraction
• Starts with identification of all common
  characteristics of a set of elements
• All objects belong to a class
• Objects represent an instance of a class
• Software objects are built from a class through
  instantiation

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Representation of a Class
CLASS NAME
Attributes
Operations ( )
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Representation of a Class
MOTORCYCLE
Color Maximum Speed Engine
Start ( ) Brake ( )
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Describing a Class

• Description of a Class
• Specification
• Definition domain
• Properties of the instances
• Implementation
• How are the specifications implemented
• Details of the operations
• Data needed to allow proper functioning
• Do we really need to know everything about
  implementation?

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Encapsulation

• The secret strength of OO methods
• Encapsulation is the hiding of implementation
  details
• Consider learning to email or learning to drive
  as examples
• Some things to be treated as a black box

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Advantages of encapsulation

• Stability
• Once the class has been studied and finalized, no
  need to change them in later use
• Portability
• Possible to reuse the class for different
  projects
• Minor changes to classes make them more and more
  reusable
• The published interface should stay the same as
  far as possible

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Levels of encapsulation

• By default, attribute values are encapsulated
  within an object
• Values cannot be manipulated from the outside
  Only operations can be triggered
• Levels (visibility)
• Private ()
• Protected ()
• Public (-)

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Relationships Between Classes

• Associations
• Objects are linked
• Classes are associated
• Represented as

(a link)
SU University
Joe Student
University
Student
(an association)
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Multiplicity

• Sometimes called the cardinalityof a
  relationship
• The number of instances of each class that are or
  can be involved in a given association

1
University
Student

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Other Examples

Driver
Bus


Customer
Product


Student
Course

1
Advisor
Student

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Roles and multiplicity
1
University
Student

0..1
University
Teacher

May be replaced by..
teacher
employer

0..1
University
Person

1
student
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Next Class

• - Class Hierarchies, Inheritance, and
  Polymorphism
• -Introduction to OOAD