Download - Case Study: Class Extraction
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Case Study: Class Extraction
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The Osbert Oglesby Case Study To get Initial Class Diagram :
The aim of entity modeling step is to : extract entity classes, determine their interrelationships, and find their attributes
Simple (yet good) way to begin this step is to use the two-stage noun extraction method
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Noun Extraction: Osbert Oglesby Case Study
Stage 1: Describe product in one paragraph:
Reports are to be generated in order to improve the effectiveness of the decision-making process for buying works of art. The reports contain buying and selling information about paintings, which are classified as masterpieces, masterworks, and other paintings
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Noun Extraction: Osbert Oglesby
Stage 2: Identify nouns in this paragraph :
Reports are to be generated in order to improve the effectiveness of the decision-making process for buying works of art. The reports contain buying and selling information about paintings, which are classified as masterpieces, masterworks, and other paintings
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Noun Extraction: Osbert Oglesby
The nouns are : report, effectiveness, process, buying, work of art,
selling, information, painting, masterpiece, masterwork
Next let’s remove some candidates : effectiveness, process and information are abstract nouns
and are therefore unlikely to be entity classes
Nouns buying and selling are derived from the verbs “buy” and “sell”; so probably will be operations of some class
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Noun Extraction: Osbert Oglesby (contd)
The nouns are : Report , effectiveness, process, buying, work
of art, selling, information, painting, masterpiece, masterwork
Noun report is more likely to be a boundary class than an entity class
Noun work of art is just a synonym for painting; so remove one of them.
This leaves four candidate entity classes: Painting Class, Masterpiece Class,
Masterwork Class, and Other Painting Class
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Second Iteration of Initial Class Diagram
Consider interrelationships between entity classes
A masterpiece is a specific type of painting, and so is a masterwork and an “other painting” :
Painting Class is therefore the base class Masterpiece Class, Masterwork Class, and
Other Painting Class are subclasses of that base class
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Second Iteration of Initial Class Diagram
Figure 12.18
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Class diagram may not reflect aspects of pricing algorithm well ?
When dealing with a masterwork : “The software product first computes the
maximum purchase price as if it were a masterpiece by the same artist”
Conclude : masterwork should have all attributes of a masterpiece (so that its maximum purchase price can be computed as if it were a masterpiece) and, in addition, it may have attributes of its own.
Third Iteration of Initial Class Diagram
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Third Iteration of the Initial Class Diagram (contd)
Figure 12.19
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Fourth Iteration of Initial Class Diagram
Another aspect of pricing algorithm not reflected in the current class diagram :
“The software product computes the coefficient of similarity between each painting for which there is an auction record and the painting under consideration for purchase”
What if anything should we change ?
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Fourth Iteration of Initial Class Diagram
Need Auctioned Painting Class to make these comparisons
Maybe, an auctioned painting must be a subclass of Painting Class ?
But a painting previously been sold at an auction somewhere else has nothing to do with paintings currently on display for sale in Osbert’s gallery
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Fourth Iteration of Initial Class Diagram
Figure 12.20
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Fourth Iteration of Initial Class Diagram
So now we have that an instance of Painting Class is either :
A painting that Osbert has bought (an instance of Gallery Painting Class), or
A painting sold at some auction (an instance of Auctioned Painting Class)
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Fifth Iteration of Initial Class Diagram
Third aspect of maximum price algorithm not yet modeled is fashionability : “compute maximum purchase price from
formula F A , where F is a constant for that artist (fashionability coefficient) …”
Fashionability or Artist Class is needed : For a painting of Other Painting Class, we
then can then use the artist instance of Fashionability Class to compute maximum price that Osbert should offer to pay
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Fifth Iteration of Initial Class Diagram
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Initial Class Diagrams ? Why was the first iteration of class
diagram so inadequate? One-paragraph description correctly did
not incorporate the pricing algorithm But algorithmic details turned out to
affect class diagram
Repeated iteration and incrementation will lead to a reasonable class diagram
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Initial Class Diagram
Next, let’s add attributes to the class diagram
Later, we’ll be extending classes with operations of that class
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Fifth Iteration of Initial Class Diagram
Figure 12.22
Osbert Application Class will contain operation that starts execution of whole software product
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The Initial Dynamic Model: The Osbert Oglesby Case Study
Dynamic modeling is third step in extracting entity classes
A statechart is constructed that reflects all operations performed by or to the software product
The operations are determined from the scenarios
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The Initial Dynamic Model: The Osbert Oglesby Case Study
Initial statechart
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The Initial Dynamic Model: The Osbert Oglesby Case Study
In state Osbert Oglesby Event Loop, one of five events can occur: buy painting selected sell painting selected print report selected update fashionability selected quit selected
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Initial Main Menu: Osbert Oglesby
Graphical user interface (GUI) “Point and click”
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Dynamic Modeling In object-oriented paradigm,
there is a dynamic model for each class, rather than for the system as a whole : However, objects in this product
never move from one class to another class
Accordingly, a dynamic model for software product as a whole is appropriate
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Extracting the Boundary Classes: The Osbert Oglesby Case Study
It is easy to extract boundary classes
Each input screen, output screen, and printed report is generally modeled by a boundary class
One screen should be adequate for all four use cases
Thus one initial boundary class User Interface Class
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Initial Boundary Classes There are three reports:
The purchases report The sales report The future trends report
Content of each report is different Each report modeled by separate boundary
class So four initial boundary classes :
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Extracting Control Classes: Osbert Oglesby Case Study
Extract control classes Each nontrivial computation modeled by a control class
In case study, there are four computations : Determine maximum price for Masterpiece Determine maximum price for Masterwork Determine maximum price for Painting Determine if there is a new trend in art purchases
There are therefore four initial control classes
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Refining the Use Cases
Refine our use cases based on class extraction as well as analysis of problem thus far
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Refining the Use Cases: Osbert Oglesby
Pricing algorithm treats three types of paintings differently
Use case Buy a Painting therefore refined into three separate use cases : Buy a Masterpiece Buy a Masterwork Buy Other Painting
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Use case Produce a Report also needs to be refined Purchases and sales report both use simple data
extraction Future trends report involves computation All three reports use their own boundary classes
Thus Produce a Report use case refined into three use cases : Produce a Purchases Report Produce a Sales Report Produce a Future Trends Report
Refining the Use Cases: Osbert Oglesby
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Third Iteration of Use-Case Diagram
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Implications for remaining UML diagrams :
Buy a Painting use case must be split into three separate descriptions
Produce a Report use case must be split into three separate descriptions
Refining the Use Cases: Osbert Oglesby
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Use Case Buy a Masterpiece
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Description of Use Case Buy a Masterpiece
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Use-Case Realization
The process of extending and refining use cases is called use-case realization
That is, we aim to flesh them out in more detail and to make them “real”
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Use-Case Realization
The realization of a specific scenario of a use case is depicted using some interaction diagram : either a sequence diagram or a collaboration diagram
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Use-Case Realization
Consider use case Buy a Masterpiece
Thus far, we have only English description of use cases
Now, let’s make it concrete. First, consider what classes involved in use
case
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Buy a Masterpiece Use Case
Class diagram (classes that enter into use case)
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Four Classes Germane to the Use Case
User Interface Class models user interface
Compute Masterpiece Price Class models computation of price Osbert should offer
Masterpiece Class The computation involves comparing the masterpiece
being considered with masterpieces that have been previously auctioned
Auctioned Painting Class These masterpieces are all instances of Auctioned
Painting Class
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Buy a Masterpiece Use Case
Seller does not interact directly with software Instead, Seller provides data that
Osbert enters into the software product
This is indicated in the note (rectangle with top right-hand corner turned over)
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Buy a Masterpiece Use Case
Scenario (one possible instance of the use case)
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Buy a Masterpiece Use Case
An executing software product uses objects, not classes
Example: A specific masterpiece is not
represented by Masterpiece Class but rather by an object, a specific instance of Masterpiece Class
Such an object is denoted in UML by : Masterpiece Class
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Buy a Masterpiece Use Case
A class diagram shows the classes in the use case and their relationships
It does not show the objects Nor the sequence of messages as they are sent from
object to object
Something more is needed .. .
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Collaboration Diagram:Buy a Masterpiece
A collaboration diagram (of the realization of the scenario of the use case)
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Buy a Masterpiece Use Case
Osbert will not approve the specification document unless he understands it
Accordingly, a written description of the collaboration diagram is needed The flow of events
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Buy a Masterpiece Use Case
Express in words the flow of events of the collaboration diagram (of the realization of scenario of the use case )
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2 Types of Interaction Diagrams
UML supports two different types of interaction diagrams : Collaboration diagram Sequence diagram
Both contain the same information (events passed among objects or sequences over time), but displayed in different ways
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Buy a Masterpiece Use Case
Sequence diagram equivalent to collaboration diagram
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Notice: Buy a Masterpiece Use Case
Sequence diagram shows that every message of scenario involves either : instance of user interface class : User
Interface Class or instance of control class : Compute
Masterpiece Price Class
Every transfer of information from object A to object B is eventually followed by a transfer in the reverse direction
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2 Types of Interaction Diagrams
Software engineers (you) can choose whether to use : A sequence diagram, or A collaboration diagram, or Both
for each scenario
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Interaction Diagrams : pros and cons ?
Sequence versus collaboration diagram ?
sequence diagram : shows flow of messages and their order unambiguously Superior when transfer of information is focus of
attention
collaboration diagram : similar to a class diagram more useful when developers concentrate on classes
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Buy a Masterpiece Use Case
The seven previous figures depict different aspects of the use case Buy a Masterpiece : They use different notations Provide different levels of detail of same activity
Why do we construct so many related artifacts?
We examine this one activity from a variety of
different perspectives to learn enough about it to ensure that the analysis workflow will be correct
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Summary : Refining Use Cases
Refine use cases using interaction diagrams From the abstract to the
implementable