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Software Engineering, CPSC-4360-01, CPSC-5360-01, Lecture 4  

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Review of Last Lecture

Introduction to Case Studies Requirement Gathering Use Case Modeling Domain Modeling / Business Modeling Activity Diagram

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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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Where are we now?

Requirement

Analysis

Design

Implement

Test

Analyze the system requirements.

Use Case Realization: Understanding how

business entities can be used to support use cases.

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Analysis: Overview

Inputs: Use Cases. Domain Model.

Activities: Draft Software Architecture. Realize Use Cases into Sequence Diagrams. Refine Domain Model into Analysis Class

Diagram.

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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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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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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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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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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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Minesweeper: System Oneclass 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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Minesweeper: System Twoclass 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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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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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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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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Minesweeper: Systems Comparison

System 2

Minesweeper

System 3

MSGui

MSStorage

Minesweeper

MSGui

MSStorage

System 1

Minesweeper

Bad Cohesion

No Coupling

Good Cohesion

Medium Coupling

Good Cohesion

Low Coupling

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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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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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UML Package Diagram

Dependency: Represent the “make use of” relationship .

Package: Collection of classes or sub-packages.

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Minesweeper: Package Diagram

Presentation

Minesweeper

Application

MSGUI

MSStorage

Storage

Corresponds to programming construct: Java: Package. C++: namespace.

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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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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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Stereotypes can be text or a graphic icon. The icon can replace the normal class box.

Class Stereotype Notation

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Use Case Realization: OverviewUse 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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Sequence Diagram UML Diagram to:

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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Sequence Diagram Notation Simpler version of the sequence diagram used in

design activity.

Actor

Class

Entities

Actors or Classes involve in interaction

Lifeline

Message

With parameterMessage ( Para )

Return

Control and possibly value are returned

Activation Bar

Time Passes

Returned value

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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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Sequence Diagram: Simple Example

class A { public void example( ) { B objB;

int result;

//Sequence diagram shows the //following interaction

result = objB.method(123);

}}

: B

method ( 123 )

: A

Returned 456

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The initial realization consists of: The Staff actor. The System. Message(s) passed between them.

Case Study 1: Use Case Realization

Display Bookings: Basic Course of Events Staff enters a date; System displays bookings for that date.

Take Display Booking use case as example:

Staff sends a message.

System returns results.

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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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Case Study 1: Sequence Diagram

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).

New class added

System Message

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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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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?)

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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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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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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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Case Study 1: Sequence Diagram Ver.2

* denotes multiple

messages

The Booking class identified in the domain model

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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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Case Study 1: Updated Class Diagram (Partial)

New class added

Operation added

New association

added

Association refined

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Refining Association As we move closer to internal representation

of the system, more technical information is needed for association.

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.

A BrName

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Refining Association: Example

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.:

Human MeeEats > 1

Domain Model

Human MeemyFood

Eats > 1

Class Diagram

class Human { Mee myFood;

void method( ) { myFood.cook( );

}}

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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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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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Case Study 1: Record Booking realization

Restaurant is responsible for maintaining all bookings. makeReservation() should be handled by Restaurant.

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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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Case Study 1: Creating New Booking Object

New ObjectConstructor Message

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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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Case Study 1: Cancel Booking Sequence Diagram

Destruction message

Lifeline Terminated

Role Name

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Case Study 1: Domain Model Refinement Continues

BookingSystem has the responsibility to remember which booking is selected.

Adds an association to record this.

Association Added

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Case Study 1: Record Arrival Realization

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Case Study 1: Record Arrival Realization

Selected Booking must be a Reservation.

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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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Case Study 1: Refining Class Hierarchy

Common interface for

all subclasses

Override in

subclass

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Case Study1: Complete Analysis Class Model

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Where are we now?

Requirement

Analysis

Design

Implement

Test

Typical Artifacts: Software Architecture Sequence Diagrams Analysis Class Diagram

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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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Reading Suggestions

Chapter 4 of [Bimlesh, Andrei, Soo; 2007] Chapter 5 of [Priestley; 2004]

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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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Thank you for your attention!

Questions?