1. Software Engineering, CPSC-CSC221, Lecture 4
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2. Review of Last Lecture Introduction to Case Studies
Requirement Gathering
Use Case Modeling
Domain Modeling / Business Modeling
Activity Diagram
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3. Overview of This Lecture
Analysis
Software Architecture
Use Case Realization
Use Case → Sequence Diagrams
Domain Model Refinement
Domain Model → Partial Class Diagram
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4. Where are we now? Analyze the system requirements.
Use Case Realization:
Understanding how business entities can be used to support use cases.
Requirement
Analysis
Design
Implement
Test
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5. Analysis: Overview Inputs: Activities: Use Cases. Domain Model.
Draft Software Architecture.
Realize Use Cases into Sequence Diagrams.
Refine Domain Model into Analysis Class Diagram.
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6. Analysis: Overview
The first bridging step from the external view (system behavior) to the internal view (system implementation).
Requirements (Use Cases) give the external view.
The Domain Model gives structural information about business entities.
The Analysis is performed to understand how the business entities can be implemented and support use cases via Use Case Realization:
Adding operations to the business entities;
Move closer to an actual class in program.
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7. Difference between Analysis & Design
Distinction is not clear between the two activities:
Usually seen as one continuous activity with no clear boundary;
Made worse by using the same UML diagrams.
Analysis:
Describes the structure of a real-world application;
Focus on requirements of system.
Design:
Describes the structure of a proposed software system;
Focus on software structure that implements the system.
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8. Difference between Analysis & Design
While analysis model refers to identifying relevant objects in the real-world system, the design model is adding specific implementation details to the underlying analysis model.
Famous phrase (page 7, [Larman, 2005]): “do the right thing (analysis), and do the thing right (design).”
Example: Flight Information System [Larman, 2005]
Analysis: finding and describing the concepts/objects: Plane, Flight, Pilot, …
Design: defining software objects and how they collaborate to fulfill the requirements: a Plane object may have a tailNumber attribute and a getFlightHistory() method, …
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9. What is Software Architecture?
High level description of the overall system:
The top-level structure of subsystems.
The role and interaction of these subsystems.
Grouping of classes to:
Improve Cohesion.
Reduce Coupling.
Cohesion:
Set of “things” that work well together.
Coupling:
Inter-Dependency between two entities.
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10. Software Architecture: Example
Take the minesweeper game as an example, and identify the high level components:
Display:
Showing the game graphically.
Application Logic:
Determining the flags, numbers.
Checking whether there are more mines, etc.
Record Storage:
Storing high scores, setting, etc.
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11. Minesweeper: System One
class Minesweeper{
public void ClickOnSquare( ){
if (square == bomb) {
gameState = dead
show dead icon write high score to file }
else if (square == number){
open neighboring squares mark squares as opened
display new board
………..// some other code
Bad Cohesion:
Display functions all over the place.
Application logic buried under other operations.
Storage function not clearly separated.
Low Coupling:
Since there is only one class, there is no inter-dependency!
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12. Minesweeper: System Two
class MSGUI {
Minesweeper msApp;
MSStorage msStore;
public void mouseClickOnSquare( ){
msApp.openSquare(..);
board =
msApp.getCurrentBoard(..);
show(board); }
public void menuExitClick( ) {
score = msApp.getHighScore( );
msStore.writeHighScore(score);
class Minesweeper {
MSStorage msStore;
public void openSquare(position){
if (square == bomb)
gameState = dead;
else if (square == number){
open neighboring squares mark squares as opened; }
public Board getCurrentBoard() { ..
public void saveCurrentBoard() {
msStore.writeBoard(..);
class MSStorage {
public void writeHighScore(..){ }
public void writeBoard(..) { } }
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13. Minesweeper: System Two
Cohesion:
Each class groups logically similar functionality together:
Class MSGUI: user interface, input/output to screen;
Class Minesweeper: computation and logic;
Class MSStorage: file operations.
Coupling:
Some interdependencies. Can be improved.
Observe that MSGUI uses MSStorage directly.
Hence, if we substitute another user interface, the high score saving functionality needs to be recorded.
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14. Minesweeper: System Three
class MSGUI {
Minesweeper msApp;
// MSStorage msStore; Not needed
public void menuExitClick( ) {
msApp.closingDown( ); }
class Minesweeper {
MSStorage msStore;
public void closingDown() {
msStore.writeHighScore(..) }
public void saveCurrentBoard() {
class MSStorage {
public void writeHighScore(..){ }
public void writeBoard(..){ } }
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15. Minesweeper: System Three
Coupling:
Reduced. MSGUI depends on Minesweeper only. Minesweeper depends on MSStorage only.
Low coupling enables easy maintenance, e.g.:
Changing MSGUI to MSTextUI would not affect the main application at all.
Can swap in another storage class, e.g., database storage, by providing the same methods.
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16. Minesweeper: Systems Comparison
Bad Cohesion
No Coupling
System 2
System 3
MSGui
Minesweeper
Minesweeper
MSGui
MSStorage
MSStorage
Good Cohesion
Medium Coupling
Good Cohesion
Low Coupling
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17. Minesweeper: Observations
Trade off between cohesion and coupling:
Improving cohesion usually implies worse (higher) coupling and vice versa.
The three categories of functionality are quite widely applicable:
User Interface.
Main Application Logic.
Storage (Persistency).
These observations help shaping Software Architecture:
splitting a system into sub-systems.
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18. The Layered Architecture
One of the oldest idea in Software Engineering.
Split into three separate layers:
Presentation Layer
User Interface.
Application Layer
The underlying logic.
Implements the functionality of system.
Storage Layer
Deals with data storage: files, database, etc.
The layers are higher level abstraction:
Each may contain several classes, or several packages (group of classes).
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19. UML Package Diagram Package: Collection of classes or sub-packages.
Dependency:
Represent the “make use of” relationship .
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20. Minesweeper: Package Diagram
Presentation
Corresponds to programming construct:
Java: Package.
C++: namespace.
MSGUI
Application
Minesweeper
Storage
MSStorage
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21. Layered Architecture: Advantages
Layers aim to insulate a system from the effects of change.
For example, user interfaces often change:
but the application layer does not use the presentation layer.
so changes to system should be restricted to presentation layer classes.
Similarly, details of persistent data storage are separated from the application logic.
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22. Analysis Class Stereotypes
Within this architecture, objects can have various typical roles:
boundary objects: interact with outside actors.
control objects: manage use case behaviour.
entity objects: maintain data.
These are represented explicitly in UML by using analysis class stereotypes.
Give extra informal information for objects.
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23. Class Stereotype Notation
Stereotypes can be text or a graphic icon.
The icon can replace the normal class box.
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24. Use Case Realization: Overview
Use case realization = implementation of the use case.
Steps:
Go through each use case.
Identify interaction using a UML sequence diagram.
Refine the domain class diagram.
Possible refinements:
Adding a new class and/or data.
Define association nagivability (direction) and role name.
Adding operation to an existing class.
Focus is on System Functionality during Analysis phase.
Ignore other layers (Presentation and Storage).
Concentrate on the Application layer.
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25. Sequence Diagram UML Diagram to: Analogy to help you understand:
Emphasize the interaction in the form of messages.
Identify the party involved in the interaction.
Outline the timing of the message.
Be mainly used in Analysis and Design activities.
Analogy to help you understand:
In OO programs, objects interact with each others via method invocation (method call).
Method invocations can be modeled as messages passing between objects:
ObjectA calls method M of ObjectB, is equivalent to
ObjectA passes message M to ObjectB.
Sequence Diagram records these message passing.
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26. Sequence Diagram Notation
Simpler version of the sequence diagram used in design activity.
Entities
Actors or Classes involve in interaction
Actor
Class
Message
With parameter
Message ( Para )
Activation Bar
Lifeline
Time Passes
Returned value
Return
Control and possibly value are returned
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27. Sequence Diagram: Explanation
Time passes from top to bottom.
Classes and actors at top:
only show those participating in this interaction.
each instance has a lifeline.
Messages shown as arrows between lifelines:
labelled with operation name and parameters.
return messages (dashed) show return of control.
activations show when the receiver has control.
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28. Sequence Diagram: Simple Example
class A {
public void example( ) {
B objB;
int result;
//Sequence diagram shows the
//following interaction
result = objB.method(123); }
: B
: A
method ( 123 )
Returned 456
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29. Case Study 1: Use Case Realization
Take Display Booking use case as example:
Display Bookings: Basic Course of Events
Staff enters a date;
System displays bookings for that date.
Staff sends a message.
System returns results.
The initial realization consists of:
The Staff actor.
The System.
Message(s) passed between them.
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30. Case Study 1: Use Case Realization
The domain model shown in Lecture 3 does not contain a System class.
The Domain Model only concentrates on entities in the real world.
A new class BookingSystem is added:
An example of domain model refinement.
Categorized as a control stereotype:
Receives a high level request from the user.
Contacts the relevant classes to fulfill the request.
Boundary stereotype is not appropriate:
Emphasizes on the use case behavior, not input/output.
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31. Case Study 1: Sequence Diagram
New class added
System Message
The display(date) message originates from outside the system (it was called by an external user - Staff):
Termed as System Messages (opposite to Internal Messages, called from an object of the system).
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32. System Sequence Diagram (SSD)
Is a Sequence Diagram that shows only system messages:
An informal subtype of sequence diagram.
Concentrates on user-system exchange only.
Graphical representation of a use case.
Serves as a starting point for a more complete sequence diagram.
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33. Case Study 1: Refining SSD?
How does the BookingSystem retrieves the bookings?
Who should keep track of the bookings?
Bad choices:
BookingSystem (why?)
Booking (why?)
BookingSystem should only handle system messages.
Booking class only records details of one booking, not a collection.
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34. Case Study 1: Refining SSD
A class to store a collection of Bookings is needed.
Adding this ability to BookingSystem is undesirable:
BookingSystem should only handle system messages.
Lower the cohesion of the class.
Booking class in domain model:
Only records details of one booking. Not a collection.
Hence, another new class is needed:
Restaurant class is a reasonable choice.
Can take care of other collections:
Tables.
Customers.
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35. Case Study 1: Sequence Diagram Ver.1
New Class
New Internal Message
UI is not the focus at this stage. A placeholder function.
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36. Case Study 1: Sequence Diagram Ver.1
The sequence diagram tells us:
Staff wants to check a booking for a particular date.
BookingSystem receives the request and asks the Restaurant to provides all bookings on that date.
Follow up:
How can Restaurant find the relevant bookings from all bookings record?
One plausible way:
Checks the date on each booking record, and collects only those that match.
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37. Case Study 1: Sequence Diagram Ver.2
* denotes multiple messages
The Booking class identified in the domain model
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38. Case Study 1: Display Booking realization
At this point, a plausible sequence of messages to achieve the use case has been identified.
The sequence diagram can be used to refine the domain model.
Several key refinements:
New Restaurant and BookingSystem classes with an association between them.
A new association from Restaurant to Booking.
Restaurant maintains links to all bookings.
Messages sent from restaurant to bookings.
Association from BookingSystem to Booking.
BookingSystem maintains links to currently displayed bookings.
If these were not stored once they had been displayed, then whenever the display was updated, the current bookings would have to be retrieved again from the Restaurant object.
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39. Case Study 1: Updated Class Diagram (Partial)
New class added
New association added
Operation added
Association refined
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40. Refining Association
As we move closer to internal representation of the system, more technical information is needed for association. A B
rName
Two possible refinements:
Add navigability: The arrow above shows only A can interact with B, but not vice versa.
Add role name: A uses rName as the reference name of B.
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41. Refining Association: Example
Human
Mee
myFood
Eats > 1
Class Diagram 1
Eats >
Human
Mee
Domain Model
The refined class diagram tells us:
Only Human is aware of the Mee, i.e., Human sends message to Mee, but not the other way around.
Human uses a reference myFood to refer to the instances of Mee, i.e.:
class Human { Mee myFood;
void method( ) { myFood.cook( );
} }
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42. Case Study 1: Display Booking realization
The Display Booking use case realization is complete at this point.
Review:
Stop at System Sequence Diagram to make sure that every exchange between user and system is modeled.
From the SSD, figure out how entities in domain model can help to fulfill the request.
Add in the appropriate messages.
Refine domain model.
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43. Case Study 1: Continuing Use Case Realization
The basic technique has already been explained.
Several Use Case realizations are chosen from Case Study 1 to highlight:
Additional notation for Sequence Diagram:
Object Creation/Deletion.
Role Name to distinguish objects of the same class.
Other possible refinement to domain model.
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44. Case Study 1: Record Booking realization
Restaurant is responsible for maintaining all bookings.
makeReservation() should be handled by Restaurant.
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45. Case Study 1: Record Booking realization
Bookings must be linked to Table and Customer:
Restaurant retrieves these with the given identifying data in booking details.
New objects shown at point of creation:
Lifeline starts from that point.
Objects created by a message arriving at the instance, i.e., constructor.
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46. Case Study 1: Creating New Booking Object
Constructor Message
New Object
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47. Case Study 1: Cancel Booking realization
A three-stage process:
Select the booking to be cancelled.
Confirm cancellation with user.
Delete the corresponding booking object.
Object deletion represented by a message with a <<destroy>> stereotype.
Lifeline terminates with an ‘X’.
Role names used to distinguish the selected object from others.
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48. Case Study 1: Cancel Booking Sequence Diagram
Destruction message
Role Name
Lifeline Terminated
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49. Case Study 1: Domain Model Refinement Continues
Association Added
BookingSystem has the responsibility to remember which booking is selected.
Adds an association to record this.
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50. Case Study 1: Record Arrival Realization
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51. Case Study 1: Record Arrival Realization
Selected Booking must be a Reservation.
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52. Case Study 1: Refining Class Diagram
Should setArrivalTime( ) be defined in Booking or in Reservation class?
On the one hand, it doesn't apply to Walk-Ins.
But a common interface to all bookings is desirable.
Define operation in Booking class.
Default implementation does nothing.
Override in Reservation class.
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53. Case Study 1: Refining Class Hierarchy
Common interface for all subclasses
Override in subclass
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54. Case Study1: Complete Analysis Class Model
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55. Where are we now? Typical Artifacts: Software Architecture
Requirement
Analysis
Design
Implement
Test
Typical Artifacts:
Software Architecture
Sequence Diagrams
Analysis Class Diagram
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56. Summary Analysis has led to:
A set of use case realizations.
A refined class diagram.
We can see how the class design is going to support the functionality of the use cases.
This gives confidence that the overall design will work.
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57. Reading Suggestions Chapter 4 of [Bimlesh, Andrei, Soo; 2007]
Chapter 5 of [Priestley; 2004]
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58. Coming up next
Object-oriented design – Chapter 5 of [Bimlesh, Andrei, Soo; 2007]
Design - Chapter 6 of [Priestley; 2004]
UML Class and Object Diagrams - Chapter 8 [Priestley; 2004]
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59. Thank you for your attention! Questions?
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