CIS224 Software Projects: Software Engineering and Research Methods

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CIS224Software Projects Software Engineering
and Research Methods

• Lecture 2b
• Introducing the Unified Modeling Language (UML)
• (Based on Fowler (2004, Chapter 1) and Stevens
  and Pooley (2006, Preface Chapter 3))

David Meredith d.meredith_at_gold.ac.uk www.titanmus
ic.com/teaching/cis224-2007-8.html
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What is UML and why should we bother with it?

• UML is a graphical notation system for designing
  and describing component-based and
  object-oriented software
• graphical modeling languages for software
  development have been around for a long time
• textual programming languages not abstract enough
  for discussing high-level design
• diagrams good at communicating design without
  lots of detail
• not a replacement for a textual programming
  language
• UML is an open standard controlled by the Object
  Management Group (OMG)
• www.uml.org
• OMG set up to build standards to support
  interoperability of object-oriented systems
• This course will focus on the current standard
  which is UML 2.0
• Familiarity with UML has become a core skill for
  software engineers

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Modes of using UML

• Sketch mode
• most common
• forward engineering make UML diagram then write
  code
• reverse engineering derive UML diagram from code
• focus on important features and leave out detail
• Blueprint mode
• emphasis on completeness
• forward engineering build detailed design for
  programmers to implement
• like architect designing a building which is
  built by construction company
• Programming language mode
• draw UML diagrams that are automatically compiled
  into executable code
• Model Driven Architecture (MDA) (Kleppe et al.,
  2003)
• standard approach to using UML as a programming
  language
• Three stages
• PIM - platform-independent model (in UML)
• PSM - platform-specific model (e.g., different
  for .NET or Java, probably also in UML)
• executable code
• Executable UML (Mellor and Balcer, 2002)
• Model compiler compiles UML directly into
  executable code without passing through PSM stage

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Using UML from two perspectives

• Software perspective
• Using UML to model software systems
• Direct mapping from UML elements to software
  elements
• Two levels of abstraction (but not clear-cut
  demarcation between them)
• Interface
• Specification
• how an object behaves (i.e., what it does)
• how we interact with an object
• Implementation
• how an object achieves its behaviour (i.e., how
  exactly it does what it does)
• Conceptual perspective
• Using UML to represent concepts in some
  non-software domain (e.g., business processes)

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We will be using UML

• from a software perspective
• at the interface level of abstraction
• in sketch and blueprint mode

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Views on a design

• When we want to express a design, we do not try
  to express all aspects of it at once
• Build different models to describe different
  aspects
• Express models as diagrams in a modelling
  language like UML

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Three basic types of model

• Use case model
• describes system from the point of view of the
  users
• Static structural model
• describes elements of the system and the
  relationships between them
• Dynamic behavioural model
• describes behaviour of the system over time

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UML 2.0 diagrams

• 13 official diagram types in UML 2.0
• Box represents a diagram type
• Open-headed arrow indicates generalization
• Will only cover a small fraction of the complete
  UML in this course, concentrating on the more
  commonly used diagram types

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Legal UML

• normative UML conforms to the standard
  specification
• conventional UML may not conform to standard
  (i.e., may be non-normative) but commonly used in
  practice
• UML rules are treated as descriptive, not
  prescriptive
• but must have well-defined meaning, even if not
  strictly legal UML
• otherwise the diagrams cannot be interpreted
  reliably

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Suppression in UML

• Anything may be suppressed in a UML diagram
• Therefore cannot assume that something is not in
  the system simply because it is not in the
  diagram!
• Unless it conflicts with what is in the diagram

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Using non-UML diagrams

• Dont need to use UML all the time!
• Sometimes non-UML diagrams are useful
• Screen flow diagrams
• Flow charts
• Decision tables
• Class-Responsibility-Collaboration (CRC) cards

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Using UML in requirements analysis

• Use case diagram to describe how people interact
  with the system
• Class diagram for building up vocabulary of
  domain
• Activity diagram to describe how humans interact
  with software in the organisation
• State diagram for describing complex life cycle
  of a concept
• Have to communicate with users and customers who
  may not be experts in software engineering
• Keep notation to a minimum
• Avoid implementational details
• Domain experts (users) must understand your
  diagrams

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Using UML in design

• Class diagrams from a software perspective
• Sequence diagrams and CRC cards for common
  scenarios
• Package diagrams for large-scale structure of
  software
• State diagrams for classes with complex
  life-cycles
• Deployment diagrams to show physical layout of
  software

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Using UML in documentation

• Use UML diagrams in sketch mode to highlight and
  illustrate interesting features
• Remember
• Comprehensiveness is the enemy of
  comprehensibility

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Using UML to study other peoples code

• Use UML in sketch mode to make graphical notes
  about the structure and behaviour of a piece of
  software or code that you are studying

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Introductory case studyInitial problem statement

• You have been contracted to develop a computer
  system for a university library. The library
  currently uses a 1960s program, written in an
  obsolete language, for some simple bookkeeping
  tasks, and a card index for user browsing. You
  are asked to build an interactive system which
  handles both of these aspects online. (Stevens
  and Pooley, 2006, p. 27)

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Clarifying the requirements

• Initial problem statement is vague
• Need a more detailed analysis before agreeing to
  tackle the problem
• Requirements engineering is difficult because
• Different users have different priorities
• Users are not good at imagining or expressing
  what an ideal system would be like
• Managers who talk to developers might not be
  users and so might not have experience of doing
  the jobs that the system is supposed to help with
  

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Requirements in more detail

• Books and journals
• Library contains books and journals
• May have several copies of a given book
• Some books are for short-term loan
• Other books can be borrowed by any library member
  for 3 weeks
• Only members of staff may borrow journals
• Ordinary library members may borrow up to 6 items
  at a time
• Members of staff may borrow up to 12 items at a
  time
• New books and journals are added to the library
  and old ones are sometimes disposed of
• Current years journals are sent away to be bound
  into hardback volumes at the end of each year

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Requirements in more detail

• Borrowing
• System must keep track of when books and journals
  are borrowed and returned
• System should produce reminders when a book is
  overdue
• In the future, users may need to be able to
  extend a loan if the item is not reserved

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Requirements in more detail

• Browsing
• Users should be able to search for a book on a
  particular topic, by a particular author, etc. in
  order to
• Check if a copy of the book is available
• Reserve a copy if it is not
• Anyone can browse in the library

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Users and tasks

• High quality system must meet its users needs
• Need to identify users of a system
• Need to identify tasks each user must carry out
  with the system
• Need to prioritise tasks in order of importance
• So that we can plan development

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Actors and use cases

• In UML
• User is modelled by an actor
• Task is modelled by a use case
• Actor
• User of a system in a particular role
• e.g., library member as book borrower
• External system which interacts with our system
• E.g., bar-code reading machine
• Use case
• Task that actor needs to perform with the help of
  our system
• E.g., borrow a copy of a book
• May involve a complex process
• E.g.
• Checking that the library member has not borrowed
  the maximum number of books
• Checking that the book can be borrowed
• May include two or more scenarios, each with a
  different type of outcome
• E.g.
• User allowed to borrow book
• User refused permission to borrow book

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Documenting a use case

• Document each use case in third person, active
  voice, e.g.
• Borrow copy of book A BookBorrower presents a
  book. The system checks that the potential
  borrower is a member of the library, and does not
  already have the maximum permitted number of
  books on loan. This maximum is 6 unless the
  member is a staff member, in which case it is 12.
  If both checks succeed, the system records that
  this library member has this copy of the book on
  loan. Otherwise, the system refuses the loan.
• In use case description, say what system must
  achieve, not exactly how it should achieve it
• E.g., dont say exactly how system stores
  information about a loan

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Use case diagram
Avoid adding use cases that the customer or users
havent told you to include!
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Using use cases to determine goals of iterations

• Decide what most important use cases are and
  provide these in early iterations
• For example
• in library case study, most important use cases
  might be
• Borrow copy of book
• Return copy of book
• Borrow journal
• Return journal
• So aim to provide these at end of first iteration

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Limited use case analysis for first iteration
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. Members of
the library can normally borrow up to six items
at a time, but members of staff may borrow up to
12 items at one time. Only staff members may
borrow journals. Borrowing The system must keep
track of when books and journals are borrowed and
returned, enforcing the rules described above.
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Identifying classes

• Classes correspond to key domain abstractions
• Domain is application area that were working in
  (e.g., the library)
• Abstraction represents only relevant important
  properties of a domain concept or thing
• Can use noun identification technique to help
  with identifying appropriate classes

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Noun identification technique

• Take a clear concise statement of requirements or
  use case descriptions
• Underline all nouns and noun phrases
• This gives us candidate classes
• Remove inappropriate classes to get initial class
  list for the system

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Example of noun identification technique

• 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. Members of
  the library can normally borrow up to six items
  at a time, but members of staff may borrow up to
  12 items at one time. Only members of staff may
  borrow journals.
• Borrowing The system must keep track of when
  books and journals are borrowed and returned,
  enforcing the rules described above.
• Initial class list
• Book, Journal, Copy, LibraryMember, (Item?),
  StaffMember

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Objects representing actors

• There isnt always an object in a system
  representing each actor (e.g., StaffMember or
  LibraryMember objects)
• Can make objects representing actors responsible
  for carrying out actions on behalf of those
  actors
• E.g., send message borrow(theCopy) to
  LibraryMember object representing Jo Bloggs when
  Jo borrows a copy of a book
• In library system, need to know how many books
  each member has on loan, so makes sense to have a
  LibraryMember object for each member

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Associations between classes

• Having identified initial class list, then
  identify important real-world relationships or
  associations between the classes
• Clarifies understanding of domain
• Reveals coupling in the system
• OK for closely-related domain objects to be
  associated since expect one class to depend on
  the other
• Examples of real-world associations in the
  library system
• a copy is a copy of a book
• a library member borrows or returns a copy
• a member of staff borrows or returns a copy
• a member of staff borrows or returns a journal

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Class model for the library system
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Class model for library system(improved version)
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Dynamic behavioral models

• Class model describes static structure of system
• Also need to describe dynamic behaviour of system
• How classes work together to provide the
  behaviour required by the use cases
• Interaction diagrams show how messages pass
  between objects to realize a use case

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Library member borrows a book

• Human library member (BookBorrower actor) comes
  to issue desk with physical copy of book
  (represented by theCopy Copy).
• Library member (represented by theLibraryMember
  LibraryMember) and copy of book identified
• borrow(theCopy) message sent to theLibraryMember.
• theLibraryMember checks that the library member
  is permitted to borrow the book (might send an
  okToBorrow() message to itself).
• If theLibraryMember can borrow the copy of the
  book, then a beBorrowed(theLibraryMember) message
  is sent to theCopy which updates itself with the
  information that it has been borrowed by
  theLibraryMember.
• It may also be necessary for the theCopy to send
  a copyBorrowed(theCopy) message to an object,
  theBook Book, which represents the book of which
  theCopy is a copy.

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Sequence diagram of Borrow copy of book use
case
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State machine diagram of Book
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Going further

• Having worked out sequence diagram and state
  machine diagram, can go back to class model and
  add attributes and operations to some of the
  classes
• Once identified how all use cases are realized,
  down to level of which messages are passed, can
  then implement the classes and complete the first
  iteration

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Summary

• UML is a graphical notation system for designing
  and describing component-based and
  object-oriented software
• can be used in sketch mode, blueprint mode or
  programming language mode
• can be used from a software perspective or a
  conceptual perspective
• three basic types of model
• use case models
• static structural models
• dynamic behavioural models
• UML standard is descriptive not prescriptive
• but must be comprehensible with well-defined
  meaning
• almost anything can be suppressed in a UML
  diagram
• UML can be used in requirements analysis, design,
  documentation and for studying legacy code
• considered a simple case study which involved
  designing a library system
• Requirements analysis using UML use case diagrams
• Using the use case analysis to determine goals of
  iterations
• Identifying classes using noun identification
  technique
• Using class model to represent classes and
  associations between them
• Using a sequence diagram to represent the dynamic
  behaviour of the system when it realizes a use
  case
• Using a state machine diagram to represent how
  the state of a class changes when it receives
  messages