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3 CS 501 Spring 2008
Design in the modified Waterfall Model
Requirements
System design
Testing
Operation & maintenance
Program design
Implementation (coding)
Acceptance & release
Feasibility study
Requirements
Design
Implementation
4 CS 501 Spring 2008
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
5 CS 501 Spring 2008
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.
6 CS 501 Spring 2008
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.
7 CS 501 Spring 2008
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.
8 CS 501 Spring 2008
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
11 CS 501 Spring 2008
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.
12 CS 501 Spring 2008
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
14 CS 501 Spring 2008
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.
21 CS 501 Spring 2008
Notation: Grouping
A package is a general-purpose mechanism for organizing elements into groups.
Business rules
23 CS 501 Spring 2008
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.
25 CS 501 Spring 2008
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.
26 CS 501 Spring 2008
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.
27 CS 501 Spring 2008
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
28 CS 501 Spring 2008
Class Inheritance Diagram
Object
Component
Container
Panel
Applet
HelloWorld
ImageObserver
interface
29 CS 501 Spring 2008
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?
30 CS 501 Spring 2008
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.
31 CS 501 Spring 2008
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.
32 CS 501 Spring 2008
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.
33 CS 501 Spring 2008
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.
34 CS 501 Spring 2008
Candidate Classes
Library the name of the systemBookJournalCopyShortTermLoan eventLibraryMemberWeek measureMemberOfLibrary repeatItem book or journalTime abstract termMemberOfStaffSystem general termRule general term
35 CS 501 Spring 2008
Relations between Classes
Book is an ItemJournal is an ItemCopy is a copy of a BookLibraryMemberItemMemberOfStaff is a LibraryMember
Is Item needed?
36 CS 501 Spring 2008
Operations
LibraryMember borrows Copy
LibraryMember returns Copy
MemberOfStaff borrows Journal
MemberOfStaff returns Journal
Item not needed yet.
37 CS 501 Spring 2008
Class Diagram
MemberOfStaff
BookCopyJournal is a copy of
1..* 1
LibraryMember
1
0..*0..12
1
on loanon loan
38 CS 501 Spring 2008
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?
39 CS 501 Spring 2008
Rough Sketch: Wholesale System
RetailStore
Warehouse
Order
Invoice
Product
Shipment
Merchant
40 CS 501 Spring 2008
Rough Sketch: Wholesale System
Warehouse
Order
Invoice
Product
MerchantRetailStore
nameaddresscontactInfofinancialInfo
Shipment
Responsibilities-track status of shipped products
Reversal
damaged()return()wrongItem()
responsibility (text field)
41 CS 501 Spring 2008
Expanding a Class: Modeling Financial Information
RetailStore
Transaction1 *association
Invoice
PaymentWhich class is responsible for the financial records for a store?
42 CS 501 Spring 2008
Modeling Invoice
Shipment
Invoice
invoiceNumber
+goodsShipped()-sendInvoice()
goodsShipped
PartsListadornments+ public- private
RetailStore???
invoiceRecord
43 CS 501 Spring 2008
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.
44 CS 501 Spring 2008
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.
45 CS 501 Spring 2008
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.
46 CS 501 Spring 2008
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.
47 CS 501 Spring 2008
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
48 CS 501 Spring 2008
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?
49 CS 501 Spring 2008
Exam Question: Attributes and Operations
WeatherStation
locationmetereologicalData
collectData()getSummary()
WeatherMap
locationdate-timegeographicDataweather
gatherData()printMap()DigitizedMap
locationgeographicData
printMap()
Or should MetereologicalData be a separate object?