CS 491 Class Notes

1
CS 491 Class Notes

• October 25, 1999

2
Announcements

• Reviews
• Class schedule for the rest of semester
• Updated project specification

3
Use Cases

• Organize your functional requirements into
  discrete events
• Elements
• Actors (entities outside the system stickmen)
• Use cases (represented as ovals)
• Relationships (between actors or between use
  cases)

4
A Simple Use Case
Withdraw money
Deposit money
Customer
Balance Inquiry
Simple ATM Machine
5
Withdraw Money

• Main flow of events The use case starts when the
  system prompts the Customer for a PIN number.
  The Customer can now enter a PIN number via the
  keypad. The Customer commits the entry by
  pressing the Enter button. The system then
  checks this PIN number to see if it is valid. If
  the PIN number is valid, the system asks the
  Customer to select a transaction type from the
  following set Withdraw Money, Deposit Money,
  Balance Inquiry. The Customer selects Withdraw
  Money. The system responds by asking for the
  amount to be withdrawn. The customer types an
  amount. The amount is then dispensed through the
  cash dispenser and the use case terminates at
  that point
• Exceptional flow of events The Customer can
  cancel a transaction at any time by pressing the
  Cancel button, thus restarting the use case. No
  changes are made to the Customers account.

6
Withdraw Money (cont.)

• Exceptional flow of events The Customer can
  clear a PIN number anytime before committing it
  and reenter a new PIN number.
• Exceptional flow of events If the Customer
  enters an invalid PIN number, the use case
  restarts. If this happens three times in a row,
  the system cancels the entire transaction,
  preventing the Customer from interacting with the
  ATM for 60 seconds.
• Exceptional flow of events If the Customer
  enters an amount to withdraw that makes the total
  withdrawals for the last 24 hours above 300, the
  Customer is told that 300 is the maximum daily
  limit for withdrawals. The Customer is
  reprompted to enter an amount to withdraw and the
  use case resumes from that point.

7
Deposit Money

• Main flow of events The use case starts when the
  system prompts the Customer for a PIN number.
  The Customer can now enter a PIN number via the
  keypad. The Customer commits the entry by
  pressing the Enter button. The system then
  checks this PIN number to see if it is valid. If
  the PIN number is valid, the system asks the
  Customer to select a transaction type from the
  following set Withdraw Money, Deposit Money,
  Balance Inquiry. The Customer selects Deposit
  Money. The system responds by asking for the
  amount to be deposited. The Customer types an
  amount. The amount is then inserted into the
  cash sucker. The system then produces a message
  that the amount was properly credited to the
  Customers account and the use case terminates at
  that point.
• Exceptional flow of events The Customer can
  cancel a transaction at any time by pressing the
  Cancel button, thus restarting the use case. No
  changes are made to the Customers account.

8
Deposit Money (cont.)

• Exceptional flow of events The Customer can
  clear a PIN number anytime before committing it
  and reenter a new PIN number.
• Exceptional flow of events If the Customer
  enters an invalid PIN number, the use case
  restarts. If this happens three times in a row,
  the system cancels the entire transaction,
  preventing the Customer from interacting with the
  ATM for 60 seconds.
• Exceptional flow of events If the Customer fails
  to insert the indicated amount into the cash
  sucker within 30 seconds, all money is returned
  to the customer with a message that the correct
  amount was not inserted. The Customer is
  reprompted for an amount to insert and the use
  case resumes at that point.

9
Balance Inquiry

• Main flow of events The use case starts when the
  system prompts the Customer for a PIN number.
  The Customer can now enter a PIN number via the
  keypad. The Customer commits the entry by
  pressing the Enter button. The system then
  checks this PIN number to see if it is valid. If
  the PIN number is valid, the system asks the
  Customer to select a transaction type from the
  following set Withdraw Money, Deposit Money,
  Balance Inquiry. The Customer selects Balance
  Inquiry. The system responds by producing the
  Customers balance on the screen, and the use
  case terminates at that point.
• Exceptional flow of events The Customer can
  cancel a transaction at any time by pressing the
  Cancel button, thus restarting the use case. No
  changes are made to the Customers account.

10
Balance Inquiry (cont.)

• Exceptional flow of events The Customer can
  clear a PIN number anytime before committing it
  and reenter a new PIN number.
• Exceptional flow of events If the Customer
  enters an invalid PIN number, the use case
  restarts. If this happens three times in a row,
  the system cancels the entire transaction,
  preventing the Customer from interacting with the
  ATM for 60 seconds.

11
Commonality Between Use Cases

• All of three of these use cases shared major
  commonality
• This commonality could be viewed as activity
  devoted to validating the customers identity.
• This customer identity validation can form a new
  use case.

12
New Use Case
Withdraw money
Customer validation
ltltincludegtgt
ltltincludegtgt
Deposit money
Customer
ltltincludegtgt
Balance Inquiry
Simple ATM Machine
13
Customer Validation

• Main flow of events The use case starts when the
  system prompts the Customer for a PIN number.
  The Customer can now enter a PIN number via the
  keypad. The Customer commits the entry by
  pressing the Enter button. The system then
  checks this PIN number to see if it is valid. If
  the PIN number is valid, the use case is
  terminated.
• Exceptional flow of events The Customer can
  cancel a transaction at any time by pressing the
  Cancel button, thus restarting the use case. No
  changes are made to the Customers account.
• Exceptional flow of events The Customer can
  clear a PIN number anytime before committing it
  and reenter a new PIN number.
• Exceptional flow of events If the Customer
  enters an invalid PIN number, the use case
  restarts. If this happens three times in a row,
  the system cancels the entire transaction,
  preventing the Customer from interacting with the
  ATM for 60 seconds.

14
Withdraw Money

• Main flow of events Include(Validate Customer).
  If the PIN number is valid, the system asks the
  Customer to select a transaction type from the
  following set Withdraw Money, Deposit Money,
  Balance Inquiry. The Customer selects Withdraw
  Money. The system responds by asking for the
  amount to be withdrawn. The customer types an
  amount. The amount is then dispensed through the
  cash dispenser and the use case terminates at
  that point
• Exceptional flow of events The Customer can
  cancel a transaction at any time by pressing the
  Cancel button, thus restarting the use case. No
  changes are made to the Customers account.
• Exceptional flow of events If the Customer
  enters an amount to withdraw that makes the total
  withdrawals for the last 24 hours above 300, the
  Customer is told that 300 is the maximum daily
  limit for withdrawals. The Customer is
  reprompted to enter an amount to withdraw and the
  use case resumes from that point.

15
Other Use Case Issues

• Use cases let you organize functionality around
  discrete events, but
• Life is rarely this simple.
• English notation tends to be a bit ambiguous.
  Alternatives for specifying behavior
• Sequencing diagrams
• State diagrams
• These alternatives can be addenda to English
  descriptions