Part V: Software Engineering
and Implementation
Lecture Note 15
Unified Modeling Language (UML)
Use Case Diagram and Sequence Diagram
Systems Analysis and Design
Kendall & Kendall
Sixth Edition
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Major Topics
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•
•
•
•
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Use Case Diagram
Use Case Diagram Syntax
The steps for creating a Use Case Diagram
Sequence Diagram
Sequence Diagram Syntax
The steps for creating a Sequence Diagram
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Object Modeling and UML
• Systems analysts use the UML to describe objectoriented systems.
• The UML uses a set of symbols to represent
graphically the various components and relationships
within a system.
• UML is mainly used to support object-oriented system
analysis and to develop object models.
• The core of UML is formed by four techniques Use
case Diagrams, Sequence Diagrams, Class Diagrams,
and StateChart Diagrams.
• In this notes, we will introduce how to draw Use Case
diagram and Sequence diagram.
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Use Case Diagram
• Use cases are the primary drivers for all the UML
diagramming techniques. A use case diagram
illustrates the main functions of a system and the
different kinds of users that interact with it.
• The Use Case diagram includes:
Actors: which are people or things
that derive value from the system, and
Use Cases: that represent the
functionality of the system.
Associations: Draw lines to
connect use cases and actors together.
System boundary: which represents
the system.
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• The actors and use cases are separated by a
system boundary and connected by lines
representing associations.
• At time, actors are specialized versions of more
general actors. Similarly, use cases can extend or
include other use cases.
• A use case may represent several paths that a user
can take while interacting with the system.
• Drawn early on in the SDLC, when SA is gathering
and defining requirements for the system.
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Five Steps Processes for Building Use Case Diagram
1.
2.
3.
4.
5.
Identify Use Case: The analyst identifies the use cases and
creates accompanying documentation, the use case report, to
describe each function in detail.
Draws the system boundary: Place a box on the use case
diagram to represent the system. place the system’s name on
the top of the box.
Adds the use cases to the diagram: Place use cases inside
the system boundary corresponding to the number of
functions that the system must perform.
Identifies the actors: A person or another system that
interacts with and derives value from the system.
Add Associations: Draw lines to connect use cases and
actors together.
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Example: This figure shows a use case diagram for a
Doctor’s Office Appointment System.
Patients, doctors, and management personnel will use
the appointment system to make appointments, record
availability, and produce schedule information.
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Symbols for Use Case Diagram
Name
Symbol
Actor
Is a person or system that derives benefit
from and is external to the system.
Represents a major piece of system
functionality.
Use Case
System
Boundary
Meaning
System Name
Represents the scope of the system.
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Use Case Relationships
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Actor
• An actor is labeled stick figures.
• Similar to an external entity found in DFDs.
• A person or another system that interacts with and
derives value from the system.
• It is not a specific user but a role that a user can play
while interacting with the system.
• An actor requests the system to perform a function or
process.
• Example : A data-entry clerk (or a nurse entering
patient information) would not be considered an actor
because he or she would fall within the scope of the
system itself.
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• Sometimes an actor plays a
specialized role of a more general
type of actor.
• Then, a specialized actor (new
patient) is placed on the model,
shown using a line with a hollow
triangle at the end of the more
general superclass of actor (patient).
• The specialized actor will inherit the
behavior of the superclass and extend
(延伸) it in some way.
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Example1: A new patient interacts with the system
in a way that is somewhat different than for a
general patient.
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Example2: This figure shows a use case diagram for
Sales System. The specialized actors (customer and
salesman ) will inherit the behavior of the more general
superclass of actor (Purchaser).
Sales System
List Product
Order Product
Purchaser
Accept Payment
Calculate
Commission
Customer
Salesman
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Use Case
• A use case, depicted by an oval in the UML, is a major
process performed by the system that benefits an actor
in some way.
• It is labeled using a descriptive verb-noun phase.
• Use case represents a major piece of system
functionality.
• It describes a sequence of actions that the system
performs to achieve an observable result of value to an
actor.
• One use case will sometimes use the functionality or
extend or include the functionality of another use case
on the diagram, and these are shown with “include”
or “extend” associations.
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Include relationship:
• The includes relationships describes the situation in
which a use case contains behaviour that is
common to more than one use case.
• In other words, the common use case is included in
the other use cases.
• It can be used to separate out a sequence of behavior
that us used in many use cases.
• It should not be used to create a hierarchical
functional decomposition of the system.
• A dotted arrow points to the common use case
indicates the includes relationship.
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Example1: An example would be a use common Pay
Student Fees that is included in Enroll in Course and
Arrange Housing, because the both cases students
must pay their fees.
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Example 2: This diagram shows the use case for Library
System. The students can borrow the books, renew the books
and return the books from the library using Library System.
Library System
Renew the books
<<include>>
<<include>>
Check borrower’s Status
Return the books
Student
<<include>>
Borrow the books
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Extent relationship:
• The extends relationship describes the situation in
which one use case possesses the behavior that
allows the new use case to handle a variation (變化)
or exception (除外) from the basic use case.
• One use case provides additional functionality
that may be required in another use case.
• There may be multiple ways of extending a use
case, which represent variations in the way that
actors interact with the use case.
• A condition can be placed next to the dependency
arrow.
• The arrow goes from the extended to the basic use
case.
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Example 1: The extended use case Student Health
Insurance extends the basic use case Pay Student
Fees.
Extended
Use
Case
Basic
Use
Case
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Example2: This figure is a use case for Car Rental
System with an “extend” association relationship and
an “include” association relationship.
Car Rental System
Pick Up Car
<<extend>>
<<include>>
Reserve
Car
Buy Insurance
Customer
<<include>>
Return Car
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Generate Bill
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Example3: The doctor’s office system has three basic functions:
make appointment, produce schedule information, record
availability. Assume every time a patient makes an appointment, he
or she is asked to confirm contact information and basic patient
information to ensure that the system always contains the must up-todate information on its patients.
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Generalization relationship:
• Represents a specialized use case to a more generalized one.
• It is represented as an unlabeled hollow arrow, with more general
use case being higher than the lower use cases.
• Example1: The Make appointment use case has been
specialized to include a use case for an OLD patient and a NEW
patient.
– The Make Old patient appointment use case inherits the
functionality of the a Make appointment use case and
extends its own functionality with the Make payment
arrangements use case.
– The Make New patient appointment use case also inherits all
of the functionality of generic Make appointment use case
and would include any functionality necessary to capture the
information needed to insert the new patient into patient
database.
– A second specialized actor, Old patient and that patient
actor is now simply a generalization of the old and new
patient actors.
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Example1:
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System Boundary
• When you create a use case diagram, the first step
is to identify the system boundary, which is
represented by a rectangle.
• The use case are enclosed within a system
boundary.
• System Boundary, which is a box that represents
the system and clearly delineates what parts of the
diagram are external or internal to it.
• The name of the system can appear either inside or
on top of the box.
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Association
• Use cases are connected to actors through
associations.
• Show the use cases that the actors interact
• A line drawn from an actor to a use case, as
relation with no direction.
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Creating a Use Case Diagrams
When diagramming a use case, start by asking the users to
• List everything the system should do for them. This can be done
using interviews, in a joint application design sessions.
• Write down who is involved with each use case and the
responsibilities or services the use case must provide to actors or
other system.
• Review the business specifications and identity the actors in the
problem domain.
• Identify the high-level events and develop the primary use case
that describe those events and how the actors initiate them.
• Review each primary use case to determine the possible variations
of flow through the use case.
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Creating a Use Case Description
• The Use Case Description are needed before creating
the use case diagram. The use case describes can be a
step-up-step breakdown of interaction between actor
and system.
• There is no standardized use case description format.
• Often the use cases are documented using a use case
document template predetermined by the
organization, which makes the use case easier to read
and provides standardized information for each use
case in the model.
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The description may include the three main areas. Some of the areas
included are optional and may not be used by all organizations.
1. Use case identifiers and initiators.
2. Steps performed.
3. Conditions, assumptions and questions.
1. Preconditions: the condition of the system before the use case
may be performed.
2. Postconditions: the state of the system after the use case has
finished.
3. Any assumptions made that would affect the method of the
use case.
4. Any outstanding (未解决) issues or questions.
5. An optional statement of priority (优先) of the use case.
6. An optional statement of risk involved in creating the use
case.
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Use case
identifiers and
initiators.
Steps performed
Conditions,
assumptions and
questions.
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Class Work
This is a use case example of student enrollment at a
university.
• The Add Student use case indicate that students could be added in
person, using Web, using a touch-tone telephone, or other
methods. The Add Student use case includes the Verify Identity
use case to verify the identity of the student.
• The Purchase Textbook use case extends the Enroll in Class use
case, and may part of a system to enroll students in an online
course.
• The Change Student Information use case allow student to
changes their own personal information. Student could not to
change grade, outstanding fees and other information. This use
case also includes the Verify Identity use case, and in this
situation, it means having the student enter a user ID and
password before gaining access to the system.
• View Student Information allows students to view their personal
information, as well as courses and grades.
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Student
Enrollment
System
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Sequence Diagram
• Sequence Diagram illustrates a succession of
interactions between classes or object instances over
time.
• Sequence Diagrams are often used to illustrate the
processing described in use case description.
• In practice, sequence diagrams are derived from use
case analysis and are used in system design to derive
the interactions, relationships and methods of the
objects in the system.
• Sequence Diagrams are used to show the overall
pattern of the activities or interactions in a use case.
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•
•
•
•
A dynamic model that shows the explicit sequence
of messages that are passed between classes in a
defined interaction, and they are very helpful for
understanding real-time specifications and for
complex scenarios.
Generic (一般的) sequence diagram shows all
possible scenarios for a use case.
A set of instance sequence diagrams depicts a
single scenario within the use case.
Each use case scenario which is a single executable
path through a use case, may create one sequence
diagram.
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Syntax for Sequence Diagram
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Elements of a Sequence Diagram
Actor and Object
•
•
•
Actors and objects that participate in the
sequence diagram are placed across the top of
the diagram using labeled rectangle.
The leftmost is the starting object and may an
actor, window, dialog box, or other user
interface.
Classes are not required to place in particular
order, but it is nice to organize them in some
logical way.
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Lifeline
• A dotted line runs vertically below each class to
denote the lifeline of the class instances over time.
• Sometimes a class creates a temporary object and
in this case an X is placed at the end of the lifeline
at the point the object is destroyed.
• When objects continue to exist in the system after
they are used in the sequence diagram, then the
lifeline continues to the bottom of the diagram.
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Focus of Control
• A vertical rectangle on the lifeline shows the focus
of control when the object is busy doing things.
• This thin rectangle box is to show when the
classed are sending and receiving messages.
• It may split into two or more concurrent (同時發生的)
lifelines to show conditionality, whereby each
separate track corresponds to a conditional branch
in the message flow.
• It also may merge together at some subsequent
point.
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Message
• It is a communication between classes. Horizontal
solid line with arrows show messages or signals
that are sent between classes.
• The arrow on the link shows which way the
message is being passed, and any argument values
for the message are placed in parentheses next to
the message’s name.
• The order messages go from the top to the bottom
of the page. Therefore, the message located higher
on the diagram means the message occur earlier in
the sequence.
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Example: Sequence diagram for Make Appointment
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• This example illustrates an instance sequence diagram to
reflect classes and messages for the scenario “manage
appointment”.
• It describes the process by which a patient creates a new
appointment, cancels or reschedules an appointment for the
doctor’s office appointment system.
• The objects in this example are: APatient, aReceptionist, Patients,
UnpaidBills, Appointments and anAppt.
objectName:
a name with colon after it represents an object.
:class
a colon with a name after it represents a class.
ObjectName::class
a name, followed by a colon and another name, represents
an object in a class.
• Patient::List represents that Patients is an instance of the List
class that contains individual patient objects.
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• A dotted line denotes the lifeline of the class
instances over time.
• LookUpPatient is a message sent from the object
aReceptionist to the object Patients, which is a
container for the current patients to determine
whether the aPatient object is a current patient.
• There are times that a message is only sent if a
condition is met and the condition is placed between
a set of [], e, g. [aPatient Exists] LookUpBills().
Conditions are shown on a sequence diagram which
models a specific scenario.
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Six Steps for Creating a Sequence Diagram
1.
2.
3.
4.
Set the context.
Identify which objects will participate.
Set the lifeline for each object.
Lay out the messages from the top to bottom of
the diagram based on the order in which they are
sent.
5. Add the focus of control to each object’s lifeline.
6. Validate the sequence diagram.
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Class Work
Apply the concepts at CD Selections Internet Sales
System:
Step1: Determine the context of the sequence
diagram. We will use a scenario from the place
order use case ( show on next page) and list the Normal
flow of Events that contains the scenario which this
sequence diagram will need to describe:
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1. Customer submits a search request to the system.
2. The System provides the Customer a list of
recommended CDs.
3. The Customer chooses one of the CDs to find out
additional information.
4. The System provides the Customer with basic
information and reviews on the CD.
5. The Customer calls the Maintain Order use case.
6. The Customer iterates over 3 through 5 until
done shopping.
7. The Customer executes the Checkout use case.
8. The Customer leaves the Web site.
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Step2:
• Identify the objects that participate in the sequence
being modeled.
• The objects that will be need to describe this
scenario are instances of the Search Request, CD
List, CD, Marking Information, Customer, Review,
Artist Information, Sample Clip , Shopping Cart
classes. We also have an actor , Customer that
interacts with the scenario.
• To complete this step, you should lay out the objects
on the sequence diagram by drawing them, from left
to right, across the diagram.
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Step 3:
• Set the lifeline for each object.
• Draw a vertical dotted line below each of the objects.
• Also, an X should be placed at the bottom of the
lifelines for aCDL and aSC since they go away at the
end of this process.
Step 4:
• Add the messages to the diagram.
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Step 5:
• Add the focus of control to each objects and actor’s
lifeline. This is done by drawing a narrow rectangle box
over the top of the lifelines to represent when the object are
sending and receiving messages.
Step 6:
Lastly, the CD Selections team validated the diagram
by ensuring that the diagram accurately and
completely represented the underlying process
associated with the Place Order use case.
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