INTRODUCTION TO OO DESIGN

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INTRODUCTION TO OO DESIGN
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INTRODUCTION TO OO DESIGN

• Why design?
• Characteristics of good design
• Object Oriented Design
• Reading
• Chapter 1 of M. Page-Jones(2000) Fundamentals of
  Object-Oriented Design in UML.
• Chapters 1 and 2 of P. Stevens (2000) Using UML
  Software engineering with objects and components.

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OO DESIGN AND SOFTWARE DEVELOPMENT

• In waterfall before implementation
• Set of UML diagrams and program stubs as output
• In Agile there is also a design phase but
  frequently merged with coding

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WHY DESIGN?

• Complex Systems
• Large systems (too complex for one person)
• Hard constraints (costs, time, space)
• Maintenance
• Changing requirements
• Need to re-use
• Invisibility. Unlike other artefacts software is
  dynamic and abstract

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WHAT ARE GOOD SYSTEMS LIKE?

• Systems that are
• Easy to modify
• Easy to understand
• Re-usable
• Pluggability
• Customisable
• Comprised of modules
• Relatively independent (low coupling)
• Well defined interface (encapsulated)
• Well defined and understood entities
  (abstraction)
• High cohesion

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OBJECT-ORIENTED DESIGN

• Current best shot at producing good systems
• Previously
• Systems looked at in terms of processes and
  information manipulated by those processes
• In OO
• Systems looked at in terms of objects (cohesive
  entities comprising processes and information)
  and their interrelations

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WHAT DOES OO MEAN?

• Discuss what are the main characteristics of OO.
• Compile a list of around 7 main characteristics.

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THE POLYGONS PROGRAM
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TRIANGLE CLASS

• Triangle
• Creates and returns a new instance of a triangle
• makeVisible
• Turns the triangle visible on the canvas
• changeColor
• Changes the color of the triangle
• changeSize
• Changes the size of the triangle
• moveUp
• Moves the triangle up one unit
• moveDown
• Moves the triangle down one unit
• moveVertical
• Moves the triangle vertically by a specific
  amount
• getArea
• Obtains the value of the triangles area

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CANVAS CLASS

• Canvas
• Creates and returns a new instance of a canvas
• add
• Adds a triangle to the canvas
• delete
• Deletes a triangle from the canvas
• setDimension
• Sets the dimension of the canvas
• getDimension
• Obtains the dimension of the canvas
• setGridUnit
• Sets the number of pixels per grid unit
• getGridUnit
• Obtains the number of pixels per grid unit

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ENCAPSULATION
Operations
changeSize
Packaging of operations and attributes into an
object type so state is accessible only via the
provided interface
changeColor
moveUp
moveDown
getArea
Variables
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IMPLEMENTATION HIDING

• Use of encapsulation to restrict visibility of
  implementation decisions internal to the objects
  structure
• Are triangle objects represented as three point
  coordinates?
• How do we make a triangle visible?
• How are the added triangles represented within a
  canvas object? As a list? As independent objects?

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STATE RETENTION AND OBJECT IDENTY
Triangle1 new Triangle
Triangle2 new Triangle
Triangle1
Triangle2
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STATE RETENTION AND OBJECT IDENTY
Triangle1 Triangle2
Triangle1
Triangle2
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MESSAGES
triangle1.changeColor(red)
target object
Argument(s)
operation name
message

• Accessors
• Mutators

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THE POLYGONS PROGRAM
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INHERITANCE
Polygon
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POLYGON CLASS

• Polygon
• Creates and returns a new instance of a polygon
• makeVisible
• Turns the polygon visible on the canvas
• changeColor
• Changes the color of the polygon
• changeSize
• Changes the size of the polygon
• moveUp
• Moves the polygon up one unit
• moveDown
• Moves the polygon down one unit
• moveVertical
• Moves the polygon vertically by a specific amount
• getArea
• Obtains the value of the polygons area

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TRIANGLE CLASS

• Triangle
• Creates and returns a new instance of a triangle
• getArea
• Obtains the value of the triangles area

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POLYMORPHISM

• Variable polymorphism
• Polygon myPolygon
• myPolygon new Triangle
• Method polymorphism
• getArea is defined in Polygon and all its
  subclasses
• Each of the subclasses overrides this method

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POLYMORPHISM AND OVERRIDING
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PROTECTED ACCESS

• Private access in the superclass may be too
  restrictive for a subclass.
• The closer inheritance relationship is supported
  by protected access.
• Protected access is more restricted than public
  access.
• Keeping fields private still recommended.
• Define protected accessors and mutators.

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ACCESS LEVELS
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SOME DESIGN PROBLEMS (CODE SMELLS)

• Overworked Method
• Method calculates and prints result
• Schizoid Class (represents more than one entity)
• Only one class for the GUI
• Method in exile
• Store calculating total of shopping basket
• Lazy class
• Message class that only displays a message
• Bastard classes

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SOME MORE DESIGN PROBLEMS

• Speculative generality
• System implements design pattern in case some
  sophisticated functionality is required in the
  future
• Middle man
• StoreText just passes requests to Store
• Frequent switch statements
• Different types of products
• Agent as class
• Customer class with selectService, requestBooking
  methods
• Inappropriate intimacy
• CustomerDB providing classes such as getXMLFile

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SUMMARY

• Good design produces software which is
• Easy to understand
• Easy to modify
• Re-usable
• Good design has
• Modularity
• Low coupling
• High cohesion
• Important OO concepts
• Encapsulation, Implementation hiding, state
  retention, object identity, messages,
  inheritance, polymorphism