Designing the system : Objectoriented design

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Designing the system Object-oriented
design
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Information hiding(1)

• Design modules items likely to change are hidden
• Change cannot affect other modules
• Example
• class JobQueue
• // instance variables
• private int queueLength // length of job
  queue
• private int queue new int25 // queue
  can contain up to 25 jobs
• // methods
• public void initializeJobQueue ()
• // body of function

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Information hiding(2)

• public void addJobToQueue (int jobNumber)
• // body of function
• public void removeJobFromQueue (int jobNumber)
• // body of function
• // class JobQueue
• Now queue and queueLength are inaccessible

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Information hiding(3)
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Object(1)

• Variables (objects) are instantiations of
  abstract data types
• Object ADT Inheritance ( Polymorphism )
• Abstract Data Types(ADT) Data Operations
• Class A specification for an arbitrary number
  of similar objects (serves as a template from
  which object can be created).
• Classical paradigm
• record_1.field_2
• Object-oriented paradigm
• thisObject.attributeB
• thisObject.methodC

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Object(2)

• Identity
• Each object is a discrete, distinguishable
  entity.
• Classification
• Objects with the same data structure and
  operations are grouped into a class. Each object
  is an instance of a class.
• Message sending
• objects communicate through the interface via
  message passing.

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Inheritance(1)

• Property of inheritance is essential feature of
  object-oriented
• Define humanBeing to be a class
• A humanBeing has attributes, such as age, height,
  gender
• Assign values to attributes when describing
  object
• Define parent to be a subclass of humanBeing
• A parent has all attributes of humanBeing, plus
  attributes of his/her own (name of oldest child,
  number of children)
• A parent inherits all attributes of humanBeing

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Inheritance(2)
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Inheritance(3)

• JABA Implementation
• class HumanBeing
• private
• int age
• float height
• enum male, female gender
• public
• // declarations of operations on HumanBeing
• // class HumanBeing
• class Parent extends HumanBeing
• private
• char nameOfOldestChild20
• int numberOfChildren
• public
• // declarations of operations on Parent
• // class Parent

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Aggregation
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Association
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Aggregation and association
Aggregation (System box CPU MemoryDisk)
System box
Mouse
CPU
Memory
Association
Keyboard
Association
Disk
Modem
Monitor
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Polymorphism and dynamic binding(1)

• Classical paradigm
• Object-oriented paradigm

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What is Object-oriented design(OOD)?

• Characteristics of OOD
• Abstractions
• Independent entities
• System functionality operations or services
  associated with each object
• Shared data areas are eliminated.
• Objects may be distributed and execute either
  sequentially or in parallel.
• OO development
• OO analysis
• OO design
• OO programming

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Advantages of OOA/OOD

• Design changes are localized.
• Unexpected interaction with other program modules
  are unlikely
• Suitable for distributed, parallel or sequential
  implementation.
• Shared data areas are reduced.
• Increases
• reusability
• maintainability
• extensibility
• understandability

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OO Approach steps Vs. Functional approach steps

• Common OO approach steps
• step1. Identify objects(attributes and methods)
• step2. Determine the relationships among objects
• step3. Design implement objects
• Functional approach steps
• step1. Identify functionality
• step2. Determine the relationships among
  functions
• step3. Design and implement functions

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Object identification

• One of main problems in OO design.
• Various proposals
• Use a grammatical analysis of a natural language
  description of a system. Objects and attributes
  are nouns, operations or services are verbs.
• Use tangible entities
• Use a behavioral approach
• Use a scenario-based analysis

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Grammatical analysis example(1)

• Hood method by European Space Agency
• The Office Information Retrieval System(ORIS)
  can file documents under some name in one or more
  indexes, retrieve documents, display and maintain
  document indexes, archive documents and destroy
  documents. The system is activated by a request
  from the user and always returns a message to the
  user indicating the success or failure of the
  request

Retrieval system
Document
User
Index
Name
Name
User command
File Retrieve Archive Destroy
Display Delete entry Add entry
Get command Put message
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Grammatical analysis example(2)

• Problems
• Different ways to reference the same object.
  Designers cannot simply assume that nouns are
  always objects and verbs are their associated
  operations.
• No support for inheritance
• incomplete formalism, etc

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Function-oriented design(1)

• Decomposing the system into a set of interacting
  functions with a centralized system state shared
  by these functions

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Function-oriented design(2)

• Activities
• Data flow design
• Structural decomposition
• Detailed design description Describe the
  entities in the design and their interfaces.
• Data-flow design
• Show how data passes through the system and is
  transformed by each system function.
• Design a sequence of functional transformations
  that convert system inputs into the required
  outputs.
• Represented by data flow graph

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Function-oriented design(3)-ATM
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Function-oriented design(4)

• Structural decomposition
• Model how functions are decomposed into
  sub-functions
• Decomposition hierarchy is represented by using
  graphical structure charts.
• Three-stage systems Input some data with
  validation and checking, process the data, then
  output the data.
• Data validation and output formatting functions
  should be subordinates to input and output
  function respectively.
• The role of functions near the top of the
  structural hierarchy may be to control a set of
  lower-level functions.

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Function-oriented design(5)

• Each node in the structure chart should have
  between two and seven subordinates
• Loosely coupled, highly cohesive

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Issues in design creation

• Modularity and levels of abstraction
• Collaborative design
• Differences in personal experience,
  understanding, and preference
• People sometimes behave differently in groups
  from the way they would behave individually.
• Designing the user interface
• Cultural Issues
• User preferences
• Concurrency
• Synchronization
• Mutual exclusion
• Design patterns and reuse

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OOT adoption by organization

• Factors influencing a companys ability to learn
  OOT
• The technologys complexity and immaturity
• Compatibility with organizational culture
• Industry competitiveness
• Guidelines for successful OOT adoption
• Select an appropriate level and pace
• Build realistic expectations Employ change agents
• Encourage learning-by-doing program
• Leverage existing technologies
• Build an OO architecture
• Continuously scan for new toolsets
• Institutionalize learned behaviors