Introduction to Class Diagrams

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Introduction to Class Diagrams

• There is nothing particularly object-oriented
  about Use Cases - they take a process-centric
  view of the problem
• We now need to turn that work on use cases into
  object-oriented work
• Classes can then be coded into Java Classes

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The OODI Methodology

• 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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Further Reading

• You can refer to Select Enterprise v6 for further
  information Help, Modelling,
  Class Modelling, Techniques, 1.
  Discovering Classes 2. Examining Use Cases
  for Classes 3. Examining Events for Classes

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Class Diagrams - Special Associations

• Generalisation - inheritance modelling
• Specialisation - inheritance modelling
• Aggregation
• These will appear as we progress with the design
  - if there are obvious superclasses/subclasses
  then great - include them but otherwise wait
  until you modelled the dynamic behaviour
• We will return to this subject to examine OO
  principles such as reuse

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Generalisation and Specialisation

• Inheritance is basically describes 'type-of'
  relationships
• Inheritance in shown in an class diagram by a
  triangle see example
• A discriminator is shown alongside the
  generalisation symbol - see Class Diagrams How
  To document
• E.g. Clerk is type-of Employee

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Aggregation

• Aggregation is used to represent 'part-of'
  relationships between classes.
• Aggregation is shown by the empty diamond in the
  class hierarchy at one end of a relationship line
• Cars are made up of steering wheels, engines,
  transmissions etc. Each of these components may
  be an object(class) in its own right. The
  special association of a car to its component
  parts is aggregation