Design Quality Metrics

1
Design Quality Metrics

• Arun Lakhotia
• University of Southwestern Louisiana
• Po Box 44330
• Lafayette, LA 70504, USA
• arun_at_cacs.usl.edu

2
Reference

• Steve McConnell, Code Complete, Microsoft Press,
  1993.
• See Chapter 5
• Keywords
• Cohesion
• Coupling

3
Aim

• How do you compare design alternatives?
• How do you critique a design?
• What is Design?
• Decomposition into functions
• Decomposition into modules
• Decomposition into classes

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Function Decomposition
5
Reasons for creating routines

• Reducing complexity
• Avoiding duplicate code
• Limiting effects of change
• Hiding sequences
• Improving performance
• Making central points of control
• Hiding data structures
• Hiding global data
• Hiding pointer operations
• Promoting code reuse
• Planning for a family of programs
• Making code reusable
• Improving portability
• Isolating complex operations
• Isolating use of nonstandard language
• Simplifying complicated boolean tests
• Modularization

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Quality of function decomposition

• May be defined in terms of
• Quality of individual functions
• Measured as cohesion
• Quality of interaction between functions
• Measured as coupling
• Desired decomposition
• High cohesion
• Low coupling

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Cohesion

• How strongly internal components are related?
• Poor/low cohesion
• A function does several unrelated activities
• Removing one activity does not affect another
  activity
• Good/High cohesion
• A function performs activities that are related
• Removing/changing one affects the other

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Acceptable cohesion

• Functional cohesion
• function performs only one activity
• Sequential cohesion
• Contains activities that are order dependent
  based on data flow
• Communicational cohesion
• Activities use the same data but are not
  otherwise related
• Temporal cohesion
• Activities to be performed at the same time
  (initialize, cleanup)

9
Unacceptable cohesion

• Procedural cohesion
• Contains activities ordered on user interaction
  or directly dependent on requirements
• Logical cohesion
• Activities are mutually exclusive, selected on a
  flag
• Coincidental cohesion
• No apparent cohesion

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Interconnections between functions

• Call
• Called
• Parameters passed
• Global variables

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Coupling

• How strongly are multiple functions related?
• Strong relation, implies high coupling, implies
  poor code
• Weak relation, implies low coupling, implies good
  code

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Acceptable coupling

• Simple data coupling
• data passed through parameters
• data passed is not structured
• Data structure coupling
• Data passed through parameters
• Data passed IS structured

13
Unacceptable coupling

• Control coupling
• Data passed is used to make control decision
• Global data coupling
• Communicate through global variables
• Pathological coupling
• One routine modifies the code or data of another
  routine