Feb. 23, 2004 CS 509 - WPI 1

CS 509Design of Software Systems

Lecture #5Monday, Feb. 23, 2004

Feb. 23, 2004 CS 509 - WPI 2

Term Project AdministrationChanges to Course MaterialQuestionsQuiz #3Review of Chapters 8 & 9:In-class Exercise:

Requirements for new design project

Class Format for Today

Feb. 23, 2004 CS 509 - WPI 3

Term Project Administration

Return Phase 2 – Functional Specifications And Journals

Return Quiz #2About 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?

Feb. 23, 2004 CS 509 - WPI 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

Feb. 23, 2004 CS 509 - WPI 5

Questions?

About what was covered last timeFrom the readingAbout the Term ProjectAnything else?

Feb. 23, 2004 CS 509 - WPI 6

Quiz #3

Chapters 7, 8, 9You have 15 minutes

Feb. 23, 2004 CS 509 - WPI 7

Chapter 8

Object Design:Reusing Pattern

Solutions

Feb. 23, 2004 CS 509 - WPI 8

Overview of Activities

Reuse (Chapter 8) Pick off-the-shelf components & design

patternsService specification (Chapter 9)

Precisely describe each class interfaceObject model restructuring (Chapter

10) Improve understandability & extensibility Optimizations to address performance

criteria (time, memory, etc.)

Feb. 23, 2004 CS 509 - WPI 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

Feb. 23, 2004 CS 509 - WPI 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

Feb. 23, 2004 CS 509 - WPI 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

Feb. 23, 2004 CS 509 - WPI 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

Feb. 23, 2004 CS 509 - WPI 13

Reuse Concepts

Application Objects vs. Solution Objects

Inheritance vs. DelegationThe Liskov Substitution PrincipleDesign PatternsApplication Frameworks

Feb. 23, 2004 CS 509 - WPI 14

Application & Solution Objects

Application domain objects come from the requirements & analysis Boundary, Control, Entity Represent concepts in the problem

domainSolution objects developed during

design Represent concepts with no

counterparts in the problem domain

Feb. 23, 2004 CS 509 - WPI 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?

Feb. 23, 2004 CS 509 - WPI 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?

Feb. 23, 2004 CS 509 - WPI 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

Feb. 23, 2004 CS 509 - WPI 18

Design Pattern Participants

Client Class Accesses the pattern

Pattern Interface Part of pattern that is visible to the

clientImplementor Class(es)

Provides low-level behavior (may be > 1)Extender Class

Provides a specialized implementation or extended behavior

Feb. 23, 2004 CS 509 - WPI 19

Design Patterns

AdapterBridgeStrategyAbstract FactoryCommandComposite

Feb. 23, 2004 CS 509 - WPI 20

Application Frameworks

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

Feb. 23, 2004 CS 509 - WPI 21

Chapter 9

Object Design:Specifying Interfaces

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Activities Overview

Identify missing attributes & operations

Specify type signatures & visibilitySpecify contracts on classes &

methods

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Interface Spec. Concepts

Participants: Class implementor Class extender Class user (client)

Types, signatures & visibilityContracts 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

classClass Extender

Develops specializations of the class

Feb. 23, 2004 CS 509 - WPI 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?

Feb. 23, 2004 CS 509 - WPI 26

Contracts on Classes, Methods

Invariants Predicate true for all instances of a class

Pre-conditions Predicate true before an operation is

invokedPost-conditions

Predicate true after an operation has completed

Purpose? Examples?

Feb. 23, 2004 CS 509 - WPI 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

Feb. 23, 2004 CS 509 - WPI 28

In-class Exercise

Requirements forHome Inventory System

Feb. 23, 2004 CS 509 - WPI 29

For Next Time

Phase 3 due (Design Documents)Read Chapter 10 – Mapping Models to

CodeCheck course web site for updates

Syllabus Reading material Term project

I will send email to the class when updated