Bài giảng Công nghệ phần mềm nâng cao - Phạm Ngọc Hùng - Đặc tả hướng đối tượng

Use Case Diagram
Đặc tả hướng đối tượng
2Using Use Case Diagrams
 Use case diagrams are used to visualize, 
specify, construct, and document the (intended) 
behavior of the system, during requirements 
capture and analysis.
 Provide a way for developers, domain experts 
and end-users to Communicate.
 Serve as basis for testing.
 Use case diagrams contain use cases, actors, 
and their relationships.
3Use Case
 Use cases specify desired behavior. 
 A use case describes “who” and “what” 
with the system in question
 A use case is a description of a set of 
sequences of actions, including variants, a 
system performs to yield an observable 
result of value to an actor.
 Each sequence represent an interaction of 
actors with the system.
name
4Specifying the Behavior of a Use Case
 Describing the flow of events within the use 
case.
 Can be done in natural language, formal 
language or pseudo-code.
 Includes: how and when the use case starts and 
ends; when the use case interacts with actors 
and what objects are exchanged; the basic flow 
and alternative flows of the behavior.
5Actors
 An actor represents a set of roles that 
users of use case play when interacting 
with these use cases.
 Actors can be human or automated 
systems.
 Actors are entities which require help from 
the system to perform their task or are 
needed to execute the system’s functions.
 Actors are not part of the system.
name
6Use Cases and Actors
 From the perspective of a given actor, a 
use case does something that is of value 
to the actor, such as calculate a result or 
change the state of an object.
 The Actors define the environments in 
which the system lives
7Example of Use Case Diagram
student
registration
updating
grades
output
generating
faculty
8Relationships between Use Cases
1. Generalization - use cases that are 
specialized versions of other use cases.
2. Include - use cases that are included as 
parts of other use cases. Enable to factor 
common behavior.
3. Extend - use cases that extend the 
behavior of other core use cases. Enable 
to factor variants.
91. Generalization
 The child use case inherits the 
behavior and meaning of the
parent use case.
 The child may add to or 
override the behavior of its parent.
parent
child
10
registration
graduate
registration
non-graduate
registration
More about Generalization
11
2. Include
 The base use case explicitly incorporates 
the behavior of another use case at a 
location specified in the base.
 The included use case never stands 
alone. It only occurs as a part of some 
larger base that includes it.
base included
>
12
More about Include
 Enables to avoid describing the same flow 
of events several times by putting the 
common behavior in a use case of its own.
updating
grades
output
generating
verifying
student id
>
>
13
3. Extend
 The base use case implicitly incorporates 
the behavior of another use case at certain 
points called extension points.
 The base use case may stand alone, but 
under certain conditions its behavior may 
be extended by the behavior of another 
use case.
base extending
>
14
More about Extend
 Enables to model optional behavior or 
branching under conditions.
Exam copy 
request
Exam-grade 
appeal 
>
15
Relationships between Actors
 Generalization.
student
non-graduate
student
graduate
student
16
Relationships between Use Cases 
and Actors
 Actors may be connected to use cases by 
associations, indicating that the actor and 
the use case communicate with one 
another using messages.
updating
grades
faculty
17
Example
place
phone call
cellular
network
user
receive
phone call
place
conference 
call
receive
additional 
call
use
scheduler
>
>
Cellular Telephone
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Update Items
Bookshop 
Worker
Ship Order
Bookshop 
Manager
Update Staff Details
Register Details
Customer
1
System Login
Update Customer Details
Add Items to ShoppingCart
Handle Order
>
>
>
Make Order
>
A More Complicate Example
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Each use case may include all or part of the following:
 Title or Reference Name - meaningful name of the UC
 Author/Date - the author and creation date
 Modification/Date - last modification and its date
 Purpose - specifies the goal to be achieved
 Overview - short description of the processes
 Cross References - requirements references
 Actors - agents participating
 Pre Conditions - must be true to allow execution
 Post Conditions - will be set when completes normally
 Normal flow of events - regular flow of activities
 Alternative flow of events - other flow of activities
 Exceptional flow of events - unusual situations
 Implementation issues - foreseen implementation problems
Use Case Description
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Example- Money Withdraw
 Use Case: Withdraw Money
 Author: ZB
 Date: 1-OCT-2004
 Purpose: To withdraw some cash from user’s bank account
 Overview: The use case starts when the customer inserts his credit 
card into the system. The system requests the user PIN. The system 
validates the PIN. If the validation succeeded, the customer can 
choose the withdraw operation else alternative 1 – validation failure 
is executed. The customer enters the amount of cash to withdraw. 
The system checks the amount of cash in the user account, its credit 
limit. If the withdraw amount in the range between the current 
amount + credit limit the system dispense the cash and prints a 
withdraw receipt, else alternative 2 – amount exceeded is executed.
 Cross References: R1.1, R1.2, R7
21
Example- Money Withdraw (cont.)
 Actors: Customer
 Pre Condition:
 The ATM must be in a state ready to accept transactions
 The ATM must have at least some cash on hand that it can 
dispense
 The ATM must have enough paper to print a receipt for at least 
one transaction
 Post Condition:
 The current amount of cash in the user account is the amount 
before the withdraw minus the withdraw amount
 A receipt was printed on the withdraw amount
 The withdraw transaction was audit in the System log file
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Example- Money Withdraw (cont.)
 Typical Course of events:
Actor Actions System Actions
1. Begins when a Customer arrives at ATM
2. Customer inserts a Credit card into ATM 3. System verifies the customer ID and 
status
5. Customer chooses “Withdraw” operation 4. System asks for an operation type
7. Customer enters the cash amount 6. System asks for the withdraw amount
8. System checks if withdraw amount is 
legal
9. System dispenses the cash
10. System deduces the withdraw amount 
from account
11. System prints a receipt
13. Customer takes the cash and the receipt 12. System ejects the cash card
23
Example- Money Withdraw (cont.)
 Alternative flow of events:
 Step 3: Customer authorization failed. Display an 
error message, cancel the transaction and eject the 
card.
 Step 8: Customer has insufficient funds in its account. 
Display an error message, and go to step 6.
 Step 8: Customer exceeds its legal amount. Display 
an error message, and go to step 6.
 Exceptional flow of events:
 Power failure in the process of the transaction before 
step 9, cancel the transaction and eject the card
24
Example- Money Withdraw (cont.)
 One method to identify use cases is actor-based:
- Identify the actors related to a system or organization.
- For each actor, identify the processes they initiate or participate in.
 A second method to identify use cases is event-based:
- Identify the external events that a system must respond to.
- Relate the events to actors and use cases.
 The following questions may be used to help identify the 
use cases for a system:
- What are tasks of each actor ?
- Will any actor create, store, change, remove, or read information in the 
system ?
- What use cases will create, store, change, remove, or read this 
information ?
- Will any actor need to inform the system about sudden, external 
changes ? 
- Does any actor need to be informed about certain occurrences in the 
system ?
- Can all functional requirements be performed by the use cases ?
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Moving on
 The “things” that “live” inside the system 
are responsible for carrying out the 
behavior the actors on the outside expect 
the system to provide.
 To implement a use case, we create a 
society of classes that work together to 
carry out the behavior of the use case.