11Systems Analysis and Design in a

Changing World, Fifth Edition

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Learning Objectives

Explain the purpose and objectives of object-oriented design

Develop component diagrams and deployment diagrams

Develop design class diagrams

Use CRC cards to define class responsibilities and collaborations

Explain the fundamental principles of object-oriented design

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Overview

This chapter is thorough explanation of how to design simple systems

Architechtural design is used to define the structure and configuration of the new system

Good object-oriented design is based on fundamental design principles

Design classes are a fundamental element in systems design

Class-Responsibility-Collaboration (CRC) cards are useful for designing simple systems

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Object-Oriented Design—The Bridge Between Analysis and Programming

Bridge between users’ requirements and new system’s programming

Object-oriented design is process by which detailed object-oriented models are built

Programmers use design to write code and test new system

User interface, network, controls, security, and database require design tasks and models

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Overview of Object-Oriented Programs

Set of objects that cooperate to accomplish result

Object contains program logic and necessary attributes in a single unit

Objects send each other messages and collaborate to support functions of main program

OO systems designer provides detail for programmers

Design class diagrams, interaction diagrams, and some state machine diagrams

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Object-Oriented Three-Layer Program

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Object-Oriented Design Processes and Models

Diagrams developed for analysis/requirements

Use case diagrams, use case descriptions and activity diagrams, domain model class diagrams, and system sequence diagrams

Diagrams developed for design

Component diagrams and Deployment diagrams

Interaction diagrams and package diagrams

Design class diagrams

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Design Models with

Their Respective

Input Models

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Object-Oriented Architectural Design

Desktop system

Enterprise-level system

Network or client/server system

Internet based system

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Differences between network and Internet systems

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Component Diagrams and Architectural Design

Component Diagram

Shows overall system architecture

API is the set of all public methods available in the component

Ports and Sockets define the interface

Frameset notation to extend UML notation

Used to describe web components

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Component Diagram Notation

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Two-layer Architectural Design of an Internet System

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Three-layer Architectural Design of an Internet system

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Sample Web Services Design

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Deployment Diagrams

Deployment diagram shows physical components of a new system

Node is a physical component

Artifact is an executable module

Artifacts are components after they have been compiled into executables

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Sample Deployment Diagram of an Internet System

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Fundamental Principles of Object-oriented Detailed Design

Design class diagrams and detailed sequence diagrams Use each other as inputs and are developed in parallel

Sequence diagrams define the interactions between objects in order to execute a use case.

Interactions are called messages

Correspond to method calls in programming language

Design Classes show attributes and method signatures

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Sample Sequence Diagram

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Sample Design Class

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Sample Java Class

Definition

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Object-oriented Design Process

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Design Class Symbols

UML does not distinguish between design class notation and domain model notation

Domain model class diagram shows conceptual classes in users’ work environment

Design class diagram specifically defines software classes

UML uses stereotype notation to categorize a model element by its characteristics

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Standard Stereotypes Found in Design Models

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Standard Design Classes

Entity – design identifier for problem domain class

Persistent class – exists after system is shut down

Control – mediates between boundary and entity classes, between the view layer and domain layer

Boundary – designed to live on system’s automation boundary, touched by users

User interface and windows classes

Data access – retrieves data from and sends data to database

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Design Class Notation Name – class name and stereotype information

Attribute visibility (private or public) – attribute name, type-expression, initial-value, property

Method signature – information needed to invoke (or call) the method Method visibility, method name, type-expression (return

parameter), method parameter list (incoming arguments)

Overloaded method – method with same name but two or more different parameter lists

Class-level method – method associated with class instead of each object (static or shared method), denoted by an underline

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Notation Used to Define a Design Class

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Design Class Definitions Overloaded method – a method with one name but

different parameter lists

Class-level method – a method associated with the class rather than an object

Class-level attribute – an attribute that contains the same value for all objects

Overridden method – a method in a subclass that overrides the parents method

Abstract class – a class that is never instantiated

Concrete class – a normal class with objects

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Sample Class Diagram with Design Classes and Inheritance

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Developing the First-cut Design Class Diagram

Elaborate the attributes

Visibility, Type cast, Initial values

Navigation Visibility

Ability to reference the methods of another object

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Navigation Visibility

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Navigation Visibility Guidelines

One-to-many with superior to subordinate. The visibility goes from the superior to the subordinate

Mandatory relationships for existence. Visibility goes from independent to dependent

Object needs information from another object. Visibility goes to object with the information

Navigational arrows may be bidirectional

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First Cut Class Diagram for RMO

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Detailed Design with CRC cards

Class-Responsibility-Collaboration

Design process

Select a single use case

Identify class with primary responsibility

Identify other classes that collaborate with primary class (become requests for service to other classes)

Identify responsibilities within each class (these become methods)

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CRC Card Notation

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CRC Card Set for Process new order

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Updated Design Class Diagram

withVisibility and

Methods

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Some Fundamental Design Principles

Encapsulation – each object is self-contained unit that includes data and methods that access data

Object reuse – designers often reuse same classes for windows components

Information hiding – data associated with object is not visible to outside world

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Some Fundamental Design Principles (continued)

Coupling – qualitative measure of how closely classes in a design class diagram are linked Number of navigation arrows in design class diagram or

messages in a sequence diagram

Loosely coupled – system is easier to understand and maintain

Cohesion – qualitative measure of consistency of functions within a single class Separation of responsibility – divide low cohesive class into

several highly cohesive classes

Highly cohesive – system is easier to understand and maintain and reuse is more likely

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Some Fundamental Design Principles(continued)

Protection from variations – parts of a system that are unlikely to change are segregated from those that will

Indirection – an intermediate class is placed between two classes to decouple them but still link them

Object responsibility – Objects are responsible for system processing

Responsibilities include knowing and doing

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Summary

Object-oriented design is the bridge between user requirements (in analysis models) and final system (constructed in programming language)

Systems design is driven by use cases, design class diagrams, and CRC Cards

Domain class diagrams are transformed into design class diagram

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Summary (continued) Object-oriented design principles must be applied

Encapsulation – data fields are placed in classes along with methods to process that data

Low coupling – connectivity between classes

High cohesion – nature of an individual class

Protection from variations – parts of a system that are unlikely to change are segregated from those that will

Indirection – an intermediate class is placed between two classes to decouple them but still link them

Separation navigation – access classes have to other classes

Three-layer design is used because maintainable