ObjectOriented Software Engineering

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Object-Oriented Software Engineering

• Object-oriented Analysis
• define the classes (objects) that represent the
  problem to be solved
• the manner in which the classes relate to and
  interact with one another
• the inner workings (attributes and operations) of
  objects
• the communication mechanisms (messages) that
  allow them to work together
• Object-oriented Design
• the definition of a multi-layered software
  architecture
• the specification of subsystems that perform
  required functions and provide infrastructure
  support
• a description of objects (classes) that form the
  building blocks of the system
• a description of the communication mechanisms
  that allow data to flow between layers,
  subsystems and objects
• in summary system design, object design, human
  interface design, data management design and task
  management design

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Object-Oriented Software Engineering

• Steps in Object Oriented Design
• 1. Identify objects
• Consider main concepts and extract the objects
  concerned. Also, interviews, documentation etc
• 2. Determine attributes
• Consider the characteristics that distinguish an
  object (its state)
• Need to prevent redundancies e.g. if we have the
  attribute, BooksOnLoan we do not need
  NumberOfBooksOnLoan as this can be calculated.
• 3. Determine the services for the attributes
• Typically, creation, updating, destruction are
  standard
• Services that provide information on the state of
  an object. These may involve some processing e.g.
  a calculation. This is of course hidden to the
  object requesting the service. (It could be a
  table look up or a calculation for all it knows)

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Object-Oriented Software Engineering

• 4. Determine the object hierarchy
• Search for relations between objects
• what inherits from what depends on the usage.
  E.g. ReferenceBook and Journal may not be
  subtypes of non-fictional since they will not
  have the service Borrow
• Advantages of the Object Oriented Approach
• It is a natural way to look at things
• Focuses on structuring the problem rather than a
  solution to it
• There is a direct mapping from the requirements
  to design to implementation. The object hierarchy
  from requirements analysis is the basis for
  design and then implementation

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Object-Oriented Software Engineering

• Flexibility - Easy to add specialisations e.g. a
  class MusicBook, as it simply inherits
  attributes and services from, say,
  Non-Fictional. New attributes or services can
  be easily added.
• Ease of Reuse.
• Traditional techniques identify functions rather
  than objects. Functions change but objects tend
  to stay the same.
• Inheritance allows new objects to be added as
  specialisations, so that class libraries can be
  created and easily added to
• Encapsulation means information hiding,
  abstraction, high cohesion, low coupling and
  therefore easy maintenance

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Object-Oriented Software Engineering

• Object-oriented Testing
• the nature of OO systems changes both testing
  strategy and testing tactics
• Review OOA and OOD models for correctness
• Review OOA and OOD models for consistency
• Unit testing class testing
• the smallest testable unit is the encapsulated
  class or object
• testing of conventional software tends to focus
  on the algorithmic detail of a module and the
  data that flows across the module interface
• class testing is driven by the operations
  encapsulated by the class and the state behaviour
  of the class

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Object-Oriented Software Engineering

• Object-oriented Testing
• Integration testing
• OO software does not have a hierarchical control
  structure, conventional top-down and bottom-up
  integration strategies have little meaning
• integrating operations one at a time into a class
  is often impossible because of the direct and
  indirect interactions of the components that make
  up the class
• the use of drivers and stubs as replacement
  operations is to be avoided when possible
• Thread- based Testing integrates the set of
  classes required to respond to one input or event
  for the system. Each thread is integrated and
  tested individually.Regression testing is applied
  to ensure that no side effects occur

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Object-Oriented Software Engineering

• Use-based Testing begins the construction of the
  system by testing those classes (called
  independent classes) that use very few (if any)
  of server (other) classes. After the independent
  classes are tested, the next layer of classes,
  called dependent classes, that use the
  independent classes are tested. This sequence
  continues until the entire system is constructed
• Validation Testing
• focuses on user visible actions and user
  recognizable outputs from the system (like
  conventional validation)
• the use-cases provide scenarios that have a high
  likelihood of uncovering errors in user
  interaction environments
• test cases may be derived from the object
  behaviour model and from event flow diagram

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Object-Oriented Modelling

• Object Models
• Object models describe the system in terms of
  object classes
• An object class is an abstraction over a set of
  objects with common attributes and the services
  (operations) provided by each object
• Various object models may be produced
• Inheritance models
• Aggregation models
• Service models

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Object-Oriented Modelling

• Object Models
• Natural ways of reflecting the real-world
  entities manipulated by the system
• More abstract entities are more difficult to
  model using this approach
• Object class identification is recognised as a
  difficult process requiring a deep understanding
  of the application domain
• Object classes reflecting domain entities are
  reusable across systems
• Object Class Notation

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Object-Oriented Modelling

• Inheritance Models
• Organise the domain object classes into a
  hierarchy
• Classes at the top of the hierarchy reflect the
  common features of all classes
• Object classes inherit their attributes and
  services from one or more super-classes. these
  may then be specialised as necessary
• Class hierarchy design is a difficult process if
  duplication in different branches is to be avoided

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Object-Oriented Modelling
Library Class Hierarchy
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Object-Oriented Modelling
User Class Hierarchy
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Object-Oriented Modelling

• Multiple Inheritance
• Rather than inheriting the attributes and
  services from a single parent class, a system
  which supports multiple inheritance allows object
  classes to inherit from several super-classes
• Can lead to semantic conflicts where
  attributes/services with the same name in
  different super-classes have different semantics
• Makes class hierarchy reorganisation more complex

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Object-Oriented Modelling
Multiple inheritance
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Object-Oriented Modelling

• Object Aggregation
• Aggregation model shows how classes which are
  collections are composed of other classes
• Similar to the part-of relationship in semantic
  data models
• Object Assocition
• shows how classes are interrealated, e.g.
  Lecturer teaches Module, 1M.
• Class Diagram
• shows inheritance, association, aggregation and
  multiplicity between classes

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