CS 509 Design of Software Systems

1
CS 509Design of Software Systems

• Lecture 5
• Monday, Feb. 23, 2004

2
Class Format for Today

• Term Project Administration
• Changes to Course Material
• Questions
• Quiz 3
• Review of Chapters 8 9
• In-class Exercise
• Requirements for new design project

3
Term Project Administration

• Return Phase 2 Functional Specifications
• And Journals
• Return Quiz 2
• About Phase 3
• How is the project progressing?
• Can we continue as planned?
• System Object Design for CTS due next week
• Have you started object design, or do you prefer
  to just focus on system design?

4
Changes to Course Material

• Next week start on a new project to design
• Home Inventory System (thanks to Michael)
• Discuss requirements later tonight
• Presentations or 2 phases for last month?
• Readings lecture topics
• Read Chapter 10 for next week
• New assignments will be posted to web site
• Discuss possible topics for CS562

5
Questions?

• About what was covered last time
• From the reading
• About the Term Project
• Anything else?

6
Quiz 3

• Chapters 7, 8, 9
• You have 15 minutes

7
Chapter 8

• Object Design
• Reusing Pattern Solutions

8
Overview of Activities

• Reuse (Chapter 8)
• Pick off-the-shelf components design patterns
• Service specification (Chapter 9)
• Precisely describe each class interface
• Object model restructuring (Chapter 10)
• Improve understandability extensibility
• Optimizations to address performance criteria
  (time, memory, etc.)

9
Purpose of Object Design

• Close the gap between application objects and
  selected hardware/software platforms
• Identify missing functionality not covered by
  system design
• Construct precise specifications of classes,
  attributes and operations
• Produce a model from which a set of classes can
  be implemented by individual developers

10
Reuse

• Off-the-shelf components identified during system
  design need to be integrated
• Class libraries 3rd party components selected
  for basic data structures services
• Wrapping components with custom objects
• Refining them using inheritance
• Design patterns selected for solving common
  problems isolating the effects of change

11
Interface Specifications

• Subsystem services identified during system
  design specified in terms of class interfaces
• Operations Arguments
• Type signatures
• Constraints Exceptions
• Identify operations objects needed to transfer
  data among subsystems

12
Restructuring

• Transforming optimizing the object model should
  occur after classes objects have been designed
  interfaces fully specified
• Often design activities can get side-tracked by
  optimization concerns
• Focusing on interfaces, components design
  patterns results in a more flexible design model
• Focusing on optimizations first tends to produce
  design model that is rigid difficult to modify

13
Reuse Concepts

• Application Objects vs. Solution Objects
• Inheritance vs. Delegation
• The Liskov Substitution Principle
• Design Patterns
• Application Frameworks

14
Application Solution Objects

• Application domain objects come from the
  requirements analysis
• Boundary, Control, Entity
• Represent concepts in the problem domain
• Solution objects developed during design
• Represent concepts with no counterparts in the
  problem domain

15
Inheritance vs. Delegation

• Review Set/Hashtable example on pgs 310-311
• Discuss distinction between inheritance and
  delegation
• Which one is better? Why?
• Is it a general rule, or better only in this
  case?
• Are there cases where the other is better?
• How does inheritance relate to taxonomy?

16
Liskov Substitution Principle

• If an object of type S can be substituted in
  all the places where an object of type T is
  expected, then S is a subtype of T.
• Who can interpret what this means?
• Examples?

17
Intro to Design Patterns

• Purpose
• Template solutions to recurring problems
• History
• Patterns evolve over time
• Refined for maximizing reuse flexibility
• Capture Knowledge
• Document context trade-offs
• Provide guidance about when to use inheritance
  vs. delegation

18
Design Pattern Participants

• Client Class
• Accesses the pattern
• Pattern Interface
• Part of pattern that is visible to the client
• Implementor Class(es)
• Provides low-level behavior (may be gt 1)
• Extender Class
• Provides a specialized implementation or extended
  behavior

19
Design Patterns

• Adapter
• Bridge
• Strategy
• Abstract Factory
• Command
• Composite

20
Application Frameworks

• Terminology
• Hook Method
• Infrastructure Framework
• Middleware Framework
• Enterprise Application Framework
• White Box Framework
• Black Box Framework

21
Chapter 9

• Object Design
• Specifying Interfaces

22
Activities Overview

• Identify missing attributes operations
• Specify type signatures visibility
• Specify contracts on classes methods

23
Interface Spec. Concepts

• Participants
• Class implementor
• Class extender
• Class user (client)
• Types, signatures visibility
• Contracts on classes and methods
• Invariants, pre- and post-conditions

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Participants

• Class Implementor
• Realizes the class
• Class User (Client)
• Invokes operations provided by the class
• Class Extender
• Develops specializations of the class

25
Types, Signatures Visibility

• Type specifies range of valid values
• For attribute, parameters method return
• Examples?
• Signature specifies method
• Name, param types, return type
• Visibility specifies who can see what
• Private, protected, public
• What does each mean?

26
Contracts on Classes, Methods

• Invariants
• Predicate true for all instances of a class
• Pre-conditions
• Predicate true before an operation is invoked
• Post-conditions
• Predicate true after an operation has completed
• Purpose?
• Examples?

27
Object Constraint Language

• Formal language for specifying contracts
• Constraints should be specified in design
  document, but may be done more informally
• Contracts should also be documented in code,
  using comments.
• See example code on page 358

28
In-class Exercise

• Requirements for
• Home Inventory System

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For Next Time

• Phase 3 due (Design Documents)
• Read Chapter 10 Mapping Models to Code
• Check course web site for updates
• Syllabus
• Reading material
• Term project
• I will send email to the class when updated