CS 501: Software Engineering
Lecture 17
Object Oriented Design II
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CS 501 Spring 2002
Administration
Design Presentations
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The Waterfall Model
Requirements
Requirements Analysis
System design
Design
Program design
Implementation
Coding
Unit & Integration Testing
System Testing
Acceptance Testing
Operation & Maintenance
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The Unified Modeling Language
UML is a standard language for modeling software systems
• Serves as a bridge between the requirements specification
and the implementation.
• Provides a means to specify and document the design of a
software system.
• Is process and programming language independent.
• Is particularly suited to object-oriented program
development.
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Useful Texts
Grady Booch, James Rumbaugh, Ivar Jacobson, The Unified
Modeling Language. Addison-Wesley 1999.
Grady Booch, Object-Oriented Analysis and Design with
Applications, second edition. Benjamin/Cummings 1994.
Rob Pooley, Perdita Stevens, Using UML Software
Engineering with Objects and Components. Addison-Wesley
1999.
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Rational Rose
Rational Rose is a system for creating and managing
UML diagrams.
It is available on all Computer Science Department
computers.
See: http://adm/Software/purify_install.htm for
installation instructions.
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The Importance of Modeling
• A model is a simplification of reality.
• We build models so that we can better understand the
system we are developing.
• We build models of complex system because we cannot
comprehend such a system in its entirety.
Models can be informal or formal. The more complex the
project the more valuable a formal model becomes.
BRJ
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Principles of Modeling
• The choice of what models to create has a profound
influence on how a problem is attacked and how a
solution is shaped.
• Every model can be expressed at different levels of
precision.
• The best models are connected to reality.
• No single model is sufficient. Every nontrivial
system is best approached through a small set of
nearly independent models.
BRJ
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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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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
Book
Journal
Copy
ShortTermLoan
LibraryMember
Week
MemberOfLibrary
Item
Time
MemberOfStaff
System
Rule
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the name of the system
event
measure
repeat
book or journal
abstract term
general term
general term
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Relations between Classes
Book
Journal
Copy
LibraryMember
Item
MemberOfStaff
is an
is an
is a copy of a
Item
Item
Book
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
LibraryMember
1
1
on loan
on loan
0..12
Journal
0..*
Copy
is a copy of
1..*
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Book
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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
Order
Merchant
Product
Warehouse
Invoice
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Shipment
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Rough Sketch: Wholesale System
RetailStore
name
address
contactInfo
financialInfo
Merchant
Warehouse
Order
Product
Reversal
Invoice
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Shipment
damaged()
return()
wrongItem()
Responsibilities
-track status of
shipped products
responsibility
(text field)
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Expanding a Class:
Modeling Financial Information
RetailStore
association
1
* Transaction
Which class is
responsible for the
financial records for
a store?
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Payment
Invoice
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Modeling Invoice
Shipment
???
RetailStore
invoiceRecord
goodsShipped
Invoice
invoiceNumber
PartsList
adornments
+goodsShipped()
+ public
-sendInvoice()
- private
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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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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:
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•
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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Actor and Use Case Diagram
• An actor is a user of a system in a
particular role.
BookBorrower
Borrow book
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An actor can be human or an external
system.
• A use case is a a task that an actor
needs to perform with the help of the
system.
CS 501 Spring 2002
Use Cases and Actors
• A scenario is an instance of a use case
• Actor is role, not an individual
(e.g., librarian can have many roles)
• Actor must be a "beneficiary" of the use case
(e.g., not librarian who processes book when borrowed)
In UML, the system boundary is the set of use cases.
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Use Cases for Borrowing Books
Borrow copy
of book
BookBorrower
Return copy
of book
Reserve
book
Extend
loan
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Relationships Between Use Cases:
<<uses>>
Extend
loan
BookBorrower
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Borrow copy
of book
<<uses>>
Check for
reservation
<<uses>>
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Relationships Between Use Cases:
<<extends>>
BookBorrower
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<<extends>>
Borrow copy
of book
Refuse
loan
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Use Cases in the Development Cycle
• Use cases are a tool in requirements analysis
• Intuitive -- easy to discuss with clients
• Use cases are often hard to translate into class models
• Scenarios are useful to validate design
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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 DigitizedMap
is 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
location
metereologicalData
collectData()
getSummary()
DigitizedMap
location
geographicData
printMap()
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WeatherMap
location
date-time
geographicData
weather
gatherData()
printMap()
Or should
MetereologicalData be a
separate object?
CS 501 Spring 2002
Exam Question: Class Diagram
DigitizedMap
WeatherStation
location
metereologicalData
collectData()
getSummary()
WeatherMap
1
1...*
summary
location
date-time
geographicData
weather
gatherData()
printMap()
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