Metrics for Object Oriented Design

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Metrics for Object Oriented Design

• Shyam R. Chidamber Chris F. Kemerer
• Presented by
• Ambikadevi Damodaran.

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Introduction

• Existing OO Design metrics are subject to one or
  more criticism.
• There are lot of ongoing research for new suite
  of metrics for OO design.
• Four Major steps involved in Object Oriented
  Design are
• 1.Identifying Classes.
• 2.Identifying the semantics of classes.
• 3.Identifying relationships between Classes.
• 4.Implementation of classes.

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Different Representations of OOD

• 1.D (A,R1Rn, O1..Om)
• A is a set of Object-elements,
• R1..Rn are relations,
• O1..Om are operations on A.
• 2.Viewed as collection of things with properties.
• Important properties Coupling and Cohesion are
  derived based on the above representations.

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Metric 1. Weighted Methods Per Class(WMC)

• Let a class be C1 with methods M1..Mn. Let c1cn
  be the complexity of the methods. Then
• WMC Summation of complexity c1cn
• View Points
• 1.The Number of methods and complexity of methods
  involved is a predicator of how much time and
  effort is required to develop and maintain the
  class.
• 2.The larger the number of methods in a class the
  greater the potential impact on children, since
  children will inherit all the methods defined in
  the class.
• 3.Classes with large numbers of methods are
  likely to be more application specific, limiting
  the possibility of reuse.

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Metric 2. Depth of Inheritance(DIT)

• Depth of inheritance of the class is the DIT
  metric for the class. DIT is a measure of how
  many ancestor classes can potentially affect this
  class.
• View points
• 1.The deeper a class is in the hierarchy, the
  greater the number of methods it is likely to
  inherit making it more complex.
• 2.Deeper trees constitute greater design
  complexity, since more methods and classes are
  involved.
• 3.The deeper a particular class is in the
  hierarchy, the greater the potential reuse of
  inherited methods.

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Metric 3. Number of Children

• It is a measure of how many subclasses are going
  to inherit the methods of the parent class.
• Viewpoints
• 1.Greater the number of children, greater the
  reuse, since the inheritance is a form of reuse.
• 2.Greater the number of children, the greater the
  likelihood of improper abstraction of the parent
  class. If a class has a large number of children,
  it may be a case of misuse of sub classing.
• 3. The number of children gives an idea of the
  potential influence a class has on the design. If
  a class has a large number of children, it may
  require more testing of the methods in that
  class.

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Metric 4. Coupling between object classes

• Objects are coupled if one of them acts on the
  other.
• Viewpoints
• 1.More independent the class is, the easier it is
  to reuse.
• 2.The larger the number of couples, the
  maintenance is more difficult.
• 3.The higher the inter-object class coupling the
  testing is more rigorous.

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Metric 5. Response for a class(RFC)

• The response set of a class is a set of methods
  that can potentially be executed in response to a
  message received by an object of that class.
• View points
• 1.With more number of methods, testing and
  debugging is more complicated.
• 2.The larger the number of methods that can be
  invoked from a class, the greater the complexity
  of the class.

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Metric 6 Lack of Cohesion in Methods(LCOM)

• Lack of cohesion means, classes are not properly
  encapsulated, so it means the classes can be
  split into one or more subclasses.
• View Points
• 1.Low cohesion means increased complexity.
• 2.Low cohesion also increases the likelihood of
  errors.

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Conclusion

• 1.These metrics reflects the viewpoints of OO
  developers.
• 2.These metrics can be used as a vehicle to
  address consistency of entire application.
• 3.By applying metrics, the areas that needs more
  testing can be identified.
• 4.Any one of the metrics can be chosen depending
  on the needs of the organization.