Use Case - Example

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Use Case - Example

• University library system requirements
• Books and journals the library contains books
  and journals. It may have several copies of a
  given book. Some of the books are for short term
  loans only. All other books may be borrowed by
  any library member for three weeks. Only members
  of staff may borrow journals. Members of the
  library can normally borrow up to six items at a
  time, but members of staff can borrow up to 12
  items at a time. New books and journals arrive
  regularly, and old ones are sometimes disposed
  of. The current years journals are sent away to
  be bound into volumes at the end of each year.
• Borrowing it is essential that the system keeps
  track of when books and journals are borrowed and
  returned. The new system should also produce
  reminders when a book is overdue. There may in
  future be a requirement for uses to be able to
  extend the loan of a book if it is not reserved.
• Browsing this system should allow users to
  search for a book on a particular topic, by a
  particular author, etc., to check whether a copy
  of the book is available for loan and, if not ,to
  reserve the book. Anybody can browse in the
  library.
• P. Stevens, R. Pooley. Using UML Software
  Engineering with Objects and Components.
  Addison-Wesley, 2000.

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Book Borrower
Journal Borrower
Browser
Librarian
Use Case Diagram
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• User interface
• our task is to build the underlying system
  providing functionality which will be invoked
  through the user interface.
• separation between user interface and underlying
  system.
• Beware of making diagrams very complex
• A diagram too complex to draw by hand is
  probably also it too complex to think out
  clearly.
• Split into multiple diagrams use cases at high
  level of abstraction.
• Update catalog use case Includes all the
  librarians adding and removal of books, sending
  away of journals etc. When this functionality is
  considered in detail we may want to separate
  these out into multiple use cases.
• Short descriptions of use cases
• Do not invent requirements!
• Can be useful to make a list of questions and
  possibilities with each use case, for discussion
  with the customer.

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• Borrow copy of book
• A BookBorrower presents a book. The system
  checks that the potential borrower is a member of
  the library, and that s/he does not already have
  a maximum permitted number of books on loan. This
  maximum is six unless the member is a staff
  member in which case it is twelve. If both checks
  succeed, this system records that this library
  member has this copy of the book on loan.
  Otherwise it refuses the loan.

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Identifying classes

• Key domain abstractions
• Features and facts about the library which matter
  in the system we are building to support it.
• Noun identification
• Books and journals the library contains books
  and journals. It may have several copies of a
  given book. Some off the books are for short term
  loans only. All other books may be borrowed by
  any library member for three weeks. Members of
  the library can normally borrow up to six items
  at a time, but members of staff can borrow up to
  12 items at a time. Only members of staff may
  borrow journals.

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Candidate classes

• Discard as candidate classes
• Library
• Short term loan loan is really an event, which,
  so far as we know, is not a useful object in the
  system
• Week measure of time, not a thing.
• Item vague. Clarify as either a book/journal.
• Candidate classes
• Book
• Journal
• Copy of book
• library member
• Member of staff
• Dont need to get it absolutely right at this
  stage we are not trying to design the system,
  butt to identify the important real world objects
  within the domain of the system.

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Book
1
Is a copy of
1..
Copy
LibraryMember
borrows/returns
0..1
0..
0..
borrows/returns
0..1
MemberOfStaff
Journal
borrows/returns
0..1
0..
Initial Class model
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Book
1
Is a copy of
1..
Copy
LibraryMember
borrows/returns
0..1
0..
MemberOfStaff
Journal
borrows/returns
0..1
0..
Revised class model
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Book
1
Is a copy of
1..
Copy
LibraryMember
borrows/returns
0..1
0..
Loan
MemberOfStaff
Journal
borrows/returns
0..1
0..
Same for journal?
Further Revision?
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Interaction Diagrams

• UML uses interaction diagrams to show how
  messages pass between objects of the system to
  carry out some task
• To realize a particular use case
• Not needed for every use case.
• For a complicated use case, or to clarify which
  of several use case realizations is better

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• Library member borrows a book
• In the physical world a human library member
  will come to the issuing desk with physical copy
  of a book. Via the system interface there will be
  some means of identifying the library member and
  book copy. So we can start with a library member
  object (theLibraryMember) and a certain Copy
  object (theCopy).
• The interaction begins by a BookBorrower actor
  sending a message to theLibraryMember. The
  message will specify the service required, in
  this case to borrow theCopy say,
  borrow(theCopy). This system has to check that
  it is permitted for the theLibraryMember to
  borrow another book one of the duties of the
  library member will be to perform this check
  theLibraryMember sends itself a message, say,
  okToBorrow.
• Next we want to update the systems information
  about how many copies of the book are in the
  library, since now there is one fewer. This
  involves the book object (theBook) that is
  associated with theCopy. Suppose that
  theLibraryMember informs theCopy that it is being
  borrowed by sending it the message borrow, and
  that theCopy then sends theBook a message to say
  that a copy of the book is being borrowed, say
  borrowed. Nothing else needs to be done

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theLibraryMember LibraryMember
theCopy Copy
theBook Book
aMember BookBorrower
borrow (theCopy)
1 okToBorrow
2 borrow
2.1borrowed
Interaction shown on a Sequence Diagram