ObjectOriented Metrics EEE493 2001 References:HvV 12'4

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Object-Oriented Metrics EEE493 2001
ReferencesHvV 12.4
Royal Military College of Canada Electrical and
Computer Engineering
Major Ron Smith smith-r_at_rmc.ca 1-613-541-6000
ext. 6030

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

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Quality of Design ? Product Quality

• Product maintainability and reusability are
  functions of the quality of design
• Design quality issues include
• complexity
• size- versus structure-based
• system structure
• module dependencies (last class)
• abstraction
• procedural versus data versus control
• modularity
• cohesion versus coupling
• information hiding
• a modules secret(s)

(this class)
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Use of Standard Metrics in OO

• At the method level we could use standard
  intra-module metrics (like cyclomatic complexity)
  
• provides an assessment of the design quality
  features of the code contained within methods
• however, does not provide an assessment of the
  design quality features at the object (or class)
  level
• At the object interaction level we could you use
  standard inter-module metrics (like tree
  impurity)
• has limited benefit
• fails to measure many of the desirable quality
  features unique to OO

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Characteristics of good OO Design

• Size
• desire small, simple classes which can easily be
  reused and maintained
• Hierarchy
• in addition to good architectural hierarchy,
  desire clean class inheritance hierarchies
• Abstraction
• desire classes which clearly represent the
  essence of the problem domain while hiding the
  solution
• Cohesion
• desire both strong functional and data cohesion
• Coupling
• desire loose inter-class coupling

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The CK Metrics Suite

• 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)

CK - Chidamber and Kemerer 1994
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Example
Constructor1( )
Constructor2( )
SetSomeAttribute( )
GetSomeAttribute( )
Sort(Array a)
Sort()
Sort()
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Weighted methods per class (WMC)

• A size metric
• larger classes are more complex, take more time
  to develop / maintain and are less reusable
• Calculate the complexity for each method
• (e.g. using McCabes cyclomatic complexity)
• Normalise each complexity value
• likely that each method would have equal
  weighting
• Sum the normalised complexity values
• WMC ? ci wi
• where wi 1/method_count equal weight
• Example (Class21) WMC (11114)/5 1.6

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Depth of inheritance tree (DIT)

• The maximum length from a node to the root
• 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
• Example (Class21) DIT 2

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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
• low NOC may mean poor use of inheritance
• But as NOC increases the parents abstraction
  may be being diluted
• maintenance / reuse becomes more difficult
• the number of specialized test cases increases
• Example (Class21) NOC 1

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Coupling between object classes (CBO)

• A coupling metric
• 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
• high coupling is an indication of strong
  inter-dependencies, meaning maintainability and
  reusability will be more difficult
• Example (Class21) CBO 2 sort( )
  dependencies

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Response for a class (RFC)

• Also a coupling metric (extent of coupling vs.
  number of couples)
• 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)
• Similarly, want this number to be low

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Lack of cohesion in methods (LCOM)

• This is an inverse measure of a classs cohesion
• A 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 (class) function
• Want to keep this low
• Many other versions of LCOM have been defined

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Supplemental References

• Roger S. Pressman. Software Engineering - A
  Practitioners Approach 5th Edition, Sections
  24-3,4. McGraw-Hill, 2001. ISBN 0-07-365578-3
• S.R. Chidamber and C.F. Kemerer. A Metrics
  Suite for Object-Oriented Design. IEEE
  Transactions on Software Engineering,
  20(6)476-493,1994.

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