Practical ObjectOriented Development with UML and Java

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Practical Object-Oriented Development with UML
and Java

• Chapter 4Finding the Objects

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Building The OO Model

• Historically two distinct activities are taking
  place simultaneously
• problem analysis
• product description
• In actuality we are building a model of reality
  that captures both aspects

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The Results

• A Model that
• Organizes the data into objects and classes, and
  gives the data a structure via relationships of
  inheritance, aggregation, and association
• Organizes the services into classes or interfaces
  and gives them conceptual grouping via the
  generalization/specialization hierarchical
  structure
• Defines the external interfaces (services
  prototypes) for all the class and interface
  services.
• Specifies local functional behaviors and defines
  their external interfaces
• Captures control or global behavior
• Captures constraints (limits and rules)

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Categories Of Objects (Coad and Yourdon)

• Structure
• Other systems
• Devices
• Events
• Roles
• Locations
• Organizational units

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Categories of objects (Shlaer and Mellor)

• Tangibles
• Roles
• Incidents
• Interactions
• Specification

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Object Categories (Ross)

• People
• Places
• Things
• Organizations
• Concepts
• Events

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Identification Of Objects

• Many methodologists have their own favorite
  approaches
• All techniques have short comings
• fail to identify all objects
• identifies false objects
• Current and traditional methods are covered here.

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Using The Things To Be Modeled

• Basic Steps Are
• Identify individual or group things, such as
  persons, roles, organizations, logs, reports,
  forms, etc. in the application domain
• Identify the corresponding objects and classes
• Requires significant experience with OO to apply
  successfully

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Using Definitions of Objects and Classes

• This is a direct and effective approach in which
  the developer
• uses object abstraction,
• knowledge of the domain,
• and the definition of class to intuitively
  identify them
• Requires significant training, practice,
  intuition, and experience.

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Using Object Decomposition

• Assumes most object are composed of other objects
• Basic approach is
• Find the aggregate objects or class
• Use object decomposition to identify their
  component objects or classes

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Using Generalization

• Assumes objects are identified prior to their
  classes, that every object is an instance of some
  class, and that commonalties among objects can be
  used to generalize classes.
• Steps are
• Identify all objects
• Look for two or more objects that share the same
  attributes and services
• Generalize the common aspects to form a class
• To continue, use the method Using Subclasses

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Using Subclasses

• The steps are
• Identify classes that share common resources
• Factor out the common resources to form a super
  class, and then use inheritance for all classes
  that share these resources to form simpler
  subclasses

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Using OO Domain Analysis

• This technique assumes that an OO Domain Analysis
  has already been performed in the same problem
  domain.
• The steps are
• Analyze the results of the give OODA
• Reuse objects or classes from the OODA

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Reusing an Application Framework

• This approach assumes that at least one OODA has
  been done to create an application framework of
  reusable classes
• Steps are
• Identify one ore more relevant application
  frameworks in the same application domain
• Reuse both objects and classes from previously
  developed frameworks.

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Reusing Class Hierarchies

• This technique assumes that a reuse repository
  with relevant reusable class hierarchies has been
  developed
• Steps are
• Look for classes in the reuse repository that can
  be reused
• After identifying the classes, attempt to reuse
  the associated class hierarchies
• After modifying the classses, attempt to create
  new abstract classes by grouping common
  attributes and methods
• If the classes are parameterized, supply the
  generic formal parameters

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Reusing Individual Objects and Classes

• If a reuse repository with relevant reusable
  objects and classes has been developed, we an
  reuse specific objects and classes.
• Steps are
• Look for relevant objects and classes in the
  reuse repository that can be applied to the
  application
• If necessary, modify the objects and/or classes
• Supply generic, formal parameters to
  parameterized classes as necessary.

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Using Subassemblies

• Assumes developers are incrementally developing
  subassemblies using a recursive development
  process
• The steps are
• Identify all of the objects (classes) at the
  current level that must remain temporarily
  complext because they depend on one or more as
  yet unidentified objects and classes
• Develop a skeleton specification/design for the
  methods of the temporarily incomplete object.
• Create the appropriate child subassemblies at the
  next lower level to handle the messages for the
  incomplete object at the higher level
• Repeat all steps at the current level that has
  new subassemblies

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Using Personal Experience

• This assumes that the developer has already
  performed an analysis and can use that experience
  for this analysis
• Steps are
• Find the objects and classes that correspond to
  ones found in previous models that are in the
  same application domain.
• Modify the classes as necessary to support the
  present project

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Using Nouns

• This assumes that written documents about the
  domain exist
• Steps are
• Obtain or author a narrative English text that
  represents an informal description of the problem
  to be solved.
• Use the nouns, pronouns, and noun phrases to
  identify objects and classes
• Verbs and predicate phrases are used to identify
  services

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Using Traditional Data Flow Diagrams

• This technique exploits existing DFD to identify
  objects and classes
• Steps are
• Map each terminator on the Context Diagram to an
  object to encapsulate the interface
• Identify one class to encapsulate the interface
  of each set of similar or identical terminator
  objects
• Map each data store on the DFD to an object
• Map data stores that contain more than one data
  field to an aggregate object
• Map all or part of the data transformation
  associated with the data store to a service of
  the object and thus of the class
• Map complex data flows to an object
• Identify subtransformations associated with the
  parts of the data flow and map these
  subtransformations to services of the object.

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Using Class-Responsibility-Collaboration (CRC)
Cards

• Based on the observations that identifying
  objects and classes is a human activity that can
  be stimulated by use of small pieces of paper to
  represent objects/classes.
• Steps are
• One a CRC card, document the name of the class
  and list both the responsibilities and
  collaborators.
• Identify missing objects and/or classes that
  should be added to the existing set of CRC.

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Recommendation

• Most Novices Find That A Mixture Of Approaches
  Helps
• Using Nouns To Identify Objects and Classes
• Using CRC Cards To Promote Discussion
• Using the Using Things To Be Modeled to Elicit
  More Potential Objects