Introduction to Class Diagrams

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Introduction to Class Diagrams
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Example Class Diagram
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What does that Class Diagram tell us?
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Questions on the previous Class Diagram

• How many loans can a member make?
• How many videos can a member borrow at the same
  time?
• If a member rents out a video, which operation
  handles it?
• What do you suppose this operation may do (ie.
  its code)?
• Which class should have the ListOutstandingLoans
  operation? Why?
• Which class should have the ListLapsedMembers
  operation? Why?

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When to do Class Diagrams

• As previously mentioned there is nothing
  particularly object-oriented about Use Cases -
  they take a process-centric view of the problem
• We now need to turn that early work on use cases
  into object-oriented work as part of our life
  cycle
• Classes can be coded into Java Classes in the
  final step of the life cycle

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The OO Life Cycle

• Analysis, Design, Implementation

Object Interaction/ statediagrams
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What is a Class?

• A class is a thing or event of significance to
  the system
• A class can have attribute (data item)
  definitions and operations (behaviour)
• Individual Objects have the attribute values and
  exhibit the behaviour which was defined in the
  class to which they belong
• This is fundamentally an object oriented premise

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What is a Class Diagram?

• The Class Diagram describes the static structure
  of the classes in a system and their associations
  (relationships)
• Initially, the Class Diagram should show only
  classes and associations
• Next, the attributes and operations can be added

rents


Video
Customer
?
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Why use a Class Diagram?

• Depicts meaningful concepts and terms for the
  problem
• a sort of visual dictionary
• improve comprehension of the problem
• clarifies important information
• can lead to revision of the Use Case Model
• achieves a starting set of software classes
  which can be refined as we progress through the
  life cycle

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Building a Class Diagram

• 1. identify the Classes and then draw a
  preliminary diagram and then later.
• 2. identify the preliminary associations and add
  them to the diagram
• 3. identify the attributes and add them to the
  diagrams
• 4. identify the operations and add them to the
  diagram

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1. How to identify the Classes?

• Various strategies exist! The objects are out
  there Identification is not always easy!
• 1.1. Work through a common concept categories
  list
• 1.2. Linguistic (Abbott) analysis of the
  problem vocabulary and the Use Case scripts you
  have created
• 1.3. Create skeleton CRC cards during informal
  brainstorming meetings
• 1.4. Find and apply existing models(patterns)
  from published sources

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1.1 Common concept categories example

• Concept category Class
• physical object video
• specifications, designs video description
• transactions sales, payment
• roles customer
• organisations video store
• rules and policies LoanPolicy

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1.2 Abbott Analysis

• Abbott, R., Communications of the ACM, Program
  Design by Informal English Descriptions 1983
• make a list of the nouns or noun phrases in
  descriptions of the problem area, requirements
  document or the Use Case script
• discuss these candidate classes and normally many
  will be discarded
• rather simplistic but very useful

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1.3 CRC Cards(Class, Responsibility,
Collaboration)

• To enable a team to decide which are the
  candidate classes after nouns analysis.
• Stages
• 1. Write the name of each noun on a card.
• 2. Discuss the need for the class and discard a
  class if necessary - identify each as a definite
  class/ definite rubbish/ a doubtful class
• 3. Write a short description of the purpose of
  this class

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2. Identify Associations

• Method
• 1. make a list of the verbs in descriptions of
  the problem area, requirements document or the
  Use Case script
• 2. pair every class with every other class and
  decide if there is a meaningful relationship
• 3. On the CRC card identify Collaborators - the
  other classes that this class send / receive
  messages to / from

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Associations (contd)

• Associations are shown as lines on the class
  diagram
• Multiplicity can be shown at each end of an
  association by - any number 1 -
  only 1 2.. - 2 or more 3..14 -
  between 3 and 14 inclusive 3,5..7 - either 3
  or between 5 and 7 inclusive

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3. Identify Attributes

• Attributes in Class diagrams are just like
  attributes in entity relationship models
• Attribute type and default values can be shown on
  the class diagram
• Attributes are listed in the 2nd level of a class
  box i.e after class name

Class Name
Attributes
Operations
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4. Identify Operations

• Operations are functions that may be applied to
  or by objects in a class - use verbs
• On the CRC card identify Responsibilities - what
  must this class do and what it knows
• A method is the implementation of an operation
• Operations are listed in the lower third of the
  class box. Each operation name may be followed
  by optional details such as argument list and
  result type

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Summary QuestionsYou have had the answer to
these during the lecture

• Why do we need to create a class diagram?
• Is it possible to alter the Use Cases after doing
  the class diagram?
• Is it OK to have multiplicity of
• What is a preliminary class diagram?
• If nouns on the use case script may become
  classes, what may verbs become on the class
  diagram?
• What is an Object?
• What is an Association on a class diagram?
• What are the steps to create a Class Diagram?

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Tutorial Exercise

• Find the classes in the DVD rental shop example
• (Start by underlining the nouns on the use case
  scripts)
• Notice that you cant do class diagrams until you
  have done the Use Case Scripts bear this in
  mind for the assignment!