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CS 501: Software Engineering

Lectures 15

Object Oriented Design 1

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Administration

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Design in the modified Waterfall Model

Requirements

System design

Testing

Operation & maintenance

Program design

Implementation (coding)

Acceptance & release

Feasibility study

Requirements

Design

Implementation

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Program Design

The task of program design is to represent the software system functions in a form that can be transformed into one or more executable programs.

Given a system architecture, the program design specifies:

• programs, components, packages, classes and class hierarchies

• interfaces, protocols

• security mechanisms, operational procedures

UML models (diagrams and specifications) can be used for almost all aspects of program design

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Models: Levels of Abstraction

The complexity of a model depends on its level of abstraction:

• High-levels of abstraction show the overall system.

• Low-levels of abstraction are needed for implementation.

Two approaches:

• Model entire system at same level of abstraction, but present diagrams with different levels of detail.

• Model parts of system at different levels of abstraction.

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UML Models

A UML model consists of:

(a) A diagram. This gives a general overview of the model, showing the principal elements and how they relate to each other. A diagram is the graphical representation of a set of elements, usually rendered as a connected graph of vertices (things) and arcs (relationships).

(b) A specification. This provides details about each element of the model. Specification for models used in program design should have sufficient detail that they can be used to write code from.

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List of all Diagrams in UML

• Use case diagram shows a set of use cases and actors (a special kind of class) and their relationships. Principally used for requirements.

• Component diagram shows the organization and dependencies among a set of components. Principally used for system architecture.

• Deployment diagram shows the configuration of processing nodes and the components that live on them. Principally used for system architecture.

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List of all Diagrams in UML

These models are used principally for program design.

• Class diagram shows a set of classes, interfaces, and collaborations with their relationships.

• Object diagram shows a set of objects and their relationships.

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These models are for interactive aspects of systems. They can be used for requirements or program design.

• Interaction diagrams: set of objects and their relationships including messages that may be dispatched among them

=> Sequence diagrams: time ordering of messages

=> Collaboration diagrams: structural organization of objects that send and receive messages

• Statechart diagram shows a state machine consisting of states, transitions, events, and activities.

• Activity diagram is a statechart diagram that shows the flow from activity to activity within a system.

List of all Diagrams in UML

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Using Rational Rose for Program Design

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Class Diagrams

Window

originsize

open()close()move()display()

name

attributes (local, instance, and class variables)

operations (methods)

responsibilities (optional text)

A class is a description of a set of objects that share the same attributes, operations, relationships and semantics.

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The "Hello, World!" Example

import java.awt.Graphics;class HelloWorld extends java.applet.Applet { public void paint (Graphics g) { g.drawString ("Hello, World!", 10, 10); }}

Example from: BJR

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The HelloWorld Example

HelloWorld

paint()

class

name

operations

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Abstraction for HelloWorld

HelloWorld

paint() g.drawString ("HelloWorld", 0, 10)"

class

name

operations

annotation

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Annotation

some text note

A note is a symbol for rendering constraints and comments attached to an element or a collection of elements.

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A Typical Class Diagram

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Another Typical Design

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Specification

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Specification Fields

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General Specification Fields

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Notation: Grouping

A package is a general-purpose mechanism for organizing elements into groups.

Business rules

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Packaging Classes

applet

awt

lang

HelloWorld

java

Graphics

package

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Notation: Relationships

A dependency is a semantic relationship between two things in which a change to one may effect the semantics of the other.

0..1 *

employer employee

An association is a structural relationship that describes a set of links, a link being a connection among objects.

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Relationships

Parking

ParkingSpace

location

is_available()

1

0 ... 1

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Notation: Relationships (continued)

A generalization is a specialization/generalization relationship is which objects of the specialized element (child) are substitutable for objects of the generalized element (parent).

child parent

A realization is a semantic relationship between classifiers, wherein one classifier specifies a contract that another classifier guarantees to carry out.

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Generalization

Applet

HelloWorld

paint() Graphics

generalization

dependency

Note that the Applet and Graphics classes are shown elided, i.e., just the name is shown, not the attributes or operations.

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Notation: Interface

An interface is a collection of operations that specify a service of a class or component, i.e., the externally visible behavior of that element.

ISpelling

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Class Inheritance Diagram

Object

Component

Container

Panel

Applet

HelloWorld

ImageObserver

interface

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Modeling Classes

Given a real-life system, how do you decide what classes to use?

• What terms do the users and implementers use to describe the system? They are candidates for classes.

• Is each candidate class crisply defined?

• For each class, what is its set of responsibilities? Are the responsibilities evenly balanced among the classes?

• What attributes and operations does each class need to carry out its responsibilities?

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Coupling and Cohesion

Coupling is a measure of the dependencies between two subsystems. If two systems are strongly coupled, it is hard to modify one without modifying the other.

Cohesion is a measure of dependencies within a subsystem. If a subsystem contains many closely related functions its cohesion is high.

Aim for high cohesion within classes and weak coupling between them.

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Candidate Classes: Application Classes and Solution Classes

Application classes and solution classes:

Application classes represent application concepts. Noun identification is an effective technique to generate candidate application classes.

Solution classes represent system concepts, e.g., user interface objects, databases, etc.

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Noun Identification: A Library Example

The library contains books and journals. It may have

several copies of a given book. Some of the books are

reserved for short-term loans only. All others may be

borrowed by any library member for three weeks. Members

of the library can normally borrow up to six items at a time,

but members of staff may borrow up to 12 items at one time.

Only members of staff may borrow journals.

The system must keep track of when books and journals are

borrowed and returned and enforce the rules.

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Noun Identification: A Library Example

The library contains books and journals. It may have

several copies of a given book. Some of the books are

reserved for short-term loans only. All others may be

borrowed by any library member for three weeks. Members

of the library can normally borrow up to six items at a time,

but members of staff may borrow up to 12 items at one time.

Only members of staff may borrow journals.

The system must keep track of when books and journals are

borrowed and returned and enforce the rules.

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Candidate Classes

Library the name of the systemBookJournalCopyShortTermLoan eventLibraryMemberWeek measureMemberOfLibrary repeatItem book or journalTime abstract termMemberOfStaffSystem general termRule general term

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Relations between Classes

Book is an ItemJournal is an ItemCopy is a copy of a BookLibraryMemberItemMemberOfStaff is a LibraryMember

Is Item needed?

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Operations

LibraryMember borrows Copy

LibraryMember returns Copy

MemberOfStaff borrows Journal

MemberOfStaff returns Journal

Item not needed yet.

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Class Diagram

MemberOfStaff

BookCopyJournal is a copy of

1..* 1

LibraryMember

1

0..*0..12

1

on loanon loan

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Rough Sketch: Wholesale System

A wholesale merchant supplies retail stores from stocks of goods in a warehouse.

What classes would you use to model this business?

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Rough Sketch: Wholesale System

RetailStore

Warehouse

Order

Invoice

Product

Shipment

Merchant

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Rough Sketch: Wholesale System

Warehouse

Order

Invoice

Product

MerchantRetailStore

nameaddresscontactInfofinancialInfo

Shipment

Responsibilities-track status of shipped products

Reversal

damaged()return()wrongItem()

responsibility (text field)

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Expanding a Class: Modeling Financial Information

RetailStore

Transaction1 *association

Invoice

PaymentWhich class is responsible for the financial records for a store?

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Modeling Invoice

Shipment

Invoice

invoiceNumber

+goodsShipped()-sendInvoice()

goodsShipped

PartsListadornments+ public- private

RetailStore???

invoiceRecord

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Lessons Learned

Design is empirical. There is no single correct design. During the design process:

• Eliding: Elements are hidden to simplify the diagram

• Incomplete: Elements may be missing.

• Inconsistency: The model may not be consistent

The diagram is not the whole design. Diagrams must be backed up with specifications.

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From Candidate Classes to Completed Design

Methods used to move to final design:

Reuse: Wherever possible use existing components, or class libraries. They may need modification.

Restructuring: Change the design to improve, understandability, maintainability, etc. Techniques include merging similar classes, splitting complex classes, etc.

Optimization: Ensure that the system meets anticipated performance requirements, e.g., by changed algorithms or restructuring.

Completion: Fill all gaps, specify interfaces, etc.

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An Exam Question: Object Oriented Design

A system generates weather maps using data collected from

unattended weather stations. Each weather station collects

meteorological data and produces summaries of the data.

On request, it sends the summary information to an area

computer. The area computer uses a database of digitized

maps to generate a set of local weather maps.

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Exam Question: Noun Identification

A system generates weather maps using data collected from

unattended weather stations. Each weather station collects

meteorological data and produces summaries of the data.

On request, it sends the summary information to an area

computer. The area computer uses a database of digitized

maps to generate a set of local weather maps.

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Exam Question: Candidate Classes

System general term

WeatherMap

Data same as MeteorologicalData

WeatherStation is this a general term?

MeteorologicalData how does this relate to WeatherStation?

DataSummary how does this relate to MeteorologicalData?

AreaComputer hardware

Database general term

DigitizedMap

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Exam Question: Observations about the Candidate Classes

WeatherMap is a DigitizedMapis derived from 1...* DataSummary

WeatherStation has a set of MeteorologicalData

MeteorologicalData

DataSummary is derived from MeteorologicalData

DigitizedMap

Can Meteorological Data be an attribute of WeatherStation?Can DataSummary be combined with WeatherMap?

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Exam Question: Attributes and Operations

WeatherStation

locationmetereologicalData

collectData()getSummary()

WeatherMap

locationdate-timegeographicDataweather

gatherData()printMap()DigitizedMap

locationgeographicData

printMap()

Or should MetereologicalData be a separate object?

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Exam Question: Class Diagram

DigitizedMap

1...*1

WeatherStation

locationmetereologicalData

collectData()getSummary()

WeatherMap

locationdate-timegeographicDataweather

gatherData()printMap()

summary

*