OO Metrics EEE493 2000

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OO Metrics EEE493 2000
Royal Military College of Canada Electrical and
Computer Engineering

• Major Greg Phillips
• greg.phillips_at_rmc.ca
• 1-613-541-6000 ext. 6190

Dr. Scott Knight knight-s_at_rmc.ca 1-613-541-6000
ext. 6190
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Refs

• Pressman, R.S., Software Engineering a
  Practitioners Approach 5th Ed., McGraw-Hill,
  2001, Section 24.3 and 24.4

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Teaching Points

• Metrics for size/complexity
• Metrics for coupling
• Metrics for cohesion
• The CK metrics suite

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Review

• How can we calculate McCabe numbers for a module?
• How were McCabe numbers used in the Aurora
  mission computer software case study?

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The Intent of Object-oriented Metrics

• To better understand the quality of the product
• to assess the effectiveness of the process
• to improve the quality of the work performed at a
  project level
• i.e. no different than those derived for
  conventional software

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Nine Distinct and Measurable Characteristics of
an OO Design

• Size
• May be determined in 4 views population, volume,
  length, and functionality
• Complexity
• McCabe style complexity
• How classes of an OO design are interrelated to
  one another
• Coupling
• The physical connection between elements of an OO
  design

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Nine Distinct and Measurable Characteristics of
an OO Design

• Sufficiency
• The degree to which an abstraction possesses the
  features required of it
• e.g. a class is sufficient if it fully reflects
  all properties of the application domain it is
  modelling
• Completeness
• Similar to Sufficiency
• But, considers the what properties are required
  to fully represent the problem domain object
• Cohesion
• The extent to which the methods of a class are
  all working together to achieve a single
  well-defined purpose

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Nine Distinct and Measurable Characteristics of
an OO Design

• Primitiveness
• The degree to which a class/method is atomic
• i.e. a method cannot be constructed out of a
  sequence of other methods in the class
• Similarity
• The degree to which two or more classes are
  similar in terms of their structure, function,
  behaviour, or purpose
• Volatility
• The likelihood that change will occur
• changes in other parts of the application may
  result in mandatory adaptation of the design
  component in question

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The CK Metrics Suite

• Proposed by Chidamber and Kemerer
• One of the most widely referenced sets of OO
  metrics
• 6 metrics
• Weighted methods per class (WMC)
• Depth of inheritance tree (DIT)
• Number of children (NOC)
• Coupling between object classes (CBO)
• Response for a class (RFC)
• Lack of cohesion in methods (LCOM)

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Weighted methods per class (WMC)

• A size/complexity measure
• Calculate the complexity for each method (e.g.
  using McCabe numbers)
• Normalise the each complexity value
• Sum the normalised complexity values
• We want to keep this number low
• But how many methods in a class?

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Depth of inheritance tree (DIT)

• The maximum length from a node to the root of the
  tree
• As DIT grows the lower-level classes inherit many
  methods (making them hard to understand)(greater
  design complexity)
• But large DIT values imply that we have good
  reuse
• DIT values must be balanced

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Number of children (NOC)

• The subclasses that are immediately subordinate
  to a class in the class hierarchy
• High NOC means high reuse
• But as NOC increases the parents abstraction
  may be being diluted the number of specialized
  test cases increases

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Example

• What is DIT?
• What is NOC?

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Coupling between object classes (CBO)

• A count of
• The number of other classes which access a method
  or variable in this class, or
• contain a method or variable accessed by this
  class
• Want to keep this low

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Response for a class (RFC)

• Measures the number of methods in a class that
  can be executed in response to a message received
  by an object of that class (includes methods in
  base classes)
• Also a coupling metric (extent of coupling vs.
  number of couples)

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Lack of cohesion in methods (LCOM)

• This is an inverse measure of a classs cohesion
• The count of methods that do not have a common
  attribute with at least one other method
• High values indicate methods and attributes are
  not related to a single function
• Want to keep this low
• Many other version of LCOM have been defined

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