UML: Class diagrams

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UML Class diagrams
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Intended learning outcomes

• Define structural (static) models
• Implement modelling methodology
• Identify and use standard UML notation for
  structural modelling
• Create simple class diagrams
• Identify opportunities for refinement

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Where we are

• Develop System Request
• Understand As-Is system
• Identify improvement opportunities
• Develop the To-Be system concept
• Develop Use Cases
• Develop Structural Model
• The Class Diagram

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Structural Modeling
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Key Ideas

• A structural (static) or conceptual model
  describes the structure of the data that supports
  the business processes in an organization.
• The structure of data used in the system is
  represented through CRC cards, class diagrams,
  and object diagrams.

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Elements of a CRC card
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STRUCTURAL MODELS
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Purpose of Structural Models

• Reduce the semantic gap between the real world
  and the world of software
• Create a vocabulary for analysts and users
• Represent things, ideas, and concepts of
  importance in the application domain

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Classes

• Templates for creating instances or objects
• Concrete
• Abstract
• Typical examples
• Application domain, user interface, data
  structure, file structure, operating environment,
  document, and multimedia classes

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Attributes

• Units of information relevant to the description
  of the class
• Only attributes important to the task should be
  included

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Operations

• Action that instances/objects can take
• Focus on relevant problem-specific operations (at
  this point)

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Relationships

• Generalization
• Enables inheritance of attributes and operations
• Aggregation
• Relates parts to wholes
• Association
• Miscellaneous relationships between classes

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CLASS DIAGRAMS

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Example Conceptual Class Diagram
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Class Diagram Syntax
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The Class Diagram

• Use of Natural Language
• Discard inappropriate candidates
• From the nouns in the problem statement
• Eliminate the
• redundant
• irrelevant - those unrelated to the problem
• Vague - ill-defined boundary, very broad in scope
• attributes - things that describe an object
• operations - things that manipulate objects
• implementation constructs - extraneous to real
  world must be eliminated from analysis model,
  needed during design, but not now.

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Example problem statement for library automation
system

• Design the software to support the operation of a
  public library or group of libraries. The system
  has a number of stations and PCs for customer
  transactions. These stations are operated by
  library employees. When a book is borrowed, the
  identification card of the client is read. Next,
  the stations barcode reader reads the books
  code. When a book is returned, the identification
  card is not needed and only the books code needs
  to be read.

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Object modelling identifying classes

• Library automation system potential object list

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Object modelling keeping the right classes

• Object

• Elimination reason

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Object Modeling - The Good Classes
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Object modelling data dictionary

• Prepare a data dictionary/ description of each
  class
• Station a terminal linked to a central computer
  operated by employees with a barcode reader.

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Identify Associations

• Associations correspond to verbs in the problem
  statement
• Association any dependency between two classes
• From the problem statement
• Employee operates station
• From tacit knowledge
• Library owns computer

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Initial class diagram
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Further Specify the semantics of associations

• Include Multiplicity
• Qualifier or Qualified Associations
• Usually a name identifies an object within some
  context most names are not globally unique. The
  context combines with the name to uniquely
  identify the object.
• A Qualifier distinguishes objects on the many
  side of an association. For example, a qualifier
  bankcode could be added to distinguish the
  different banks in a consortium. Each cash card
  needs a bank code so that transactions can be
  directed to the appropriate bank.

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Identifying Attributes and operations

• Attributes
• Properties of an individual object
• Usually possessive phrases e.g. the colour of the
  car
• Identify from the problem description and domain
  knowledge
• Operations
• Define ways in which objects interact

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Identifying attributes
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Summary

• Class and object diagrams show the underlying
  structure of an object-oriented system.
• Constructing the structural model is an iterative
  process involving textual analysis,
  brainstorming objects, role playing, creating the
  diagrams, and incorporating useful patterns.