Software Metrics

• Measure aspects of process, project, product
  • Indicators
  • Provide insights
• Design Metrics
  • Design assessment tools

Design Metrics

• evaluate design quality
• evaluate reused implementation
• concentrate testing effort
• select design

Fan-in

• incoming dependencies
  • Number of external modules that depend on an internal modules
• Afferent Coupling
  • Degree of responsibility
  • the higher the more responsible it is

Fan-out

• Outgoing dependencies
  • Number of internal modules that depend on an external module
• Efferent Coupling
  • Degree of dependence
  • the higher the more dependent it is

Instability Factor

• degree of stability of a module
• Instability Factor = Efferent Coupling / (Efferent Coupling + Afferent Coupling)
  • 0 = totally stable
  • 1 = totally unstable

Stable Dependencies Principle (SDP)

The dependencies between components in a design should be in the direction of stability. A component should only depend upon components that are more stable than it is. Depend only upon components whose Instability metric is lower than yours.

Abstraction

Degree of abstractness of a module

• Number of abstract classes / number of classes
• 0 = no abstract classes, 1 = all abstract classes

Open-Closed Principle (OCP)

• A module should be open for extension, but closed for modification

  • Behavior change possible
  • NO modification to module implementation

• Violation of OCP

  • needs to change DataLogger to modify behavior

• satisfies of OCP

  • no change to DataLogger needed to modify behavior

Stable Abstractions Principle

The abstraction of a package should be proportional to its stability!

• Packages that are maximally stable should be maximally abstract
• Instable packages should be concrete
• Dependencies in the direction of abstraction

Main Sequence

Instability should increase as abstraction decreases.

Distance

• Distance from Main Sequence
  • Balance between Stability and abstractness
    • D = |A + I - 1|
    • 0 = good Balance, 1 is opposite

Zones of Exclusion

• Zone of pain
  • Highly stable and concrete
  • responsible can't be extended
• Zone of Exclusion
  • Highly instable and abstract
  • No dependents

Cyclomatic Complexity

• Logical complexity of a routine
  • E number of edges, N number of nodes, P number of predicates
  • C = E-N + 2
  • C = P+1

Weighted Methods per Class (WMC)

WMC = sum of ci, ci = cyclomatic complexity of each method

• Effort for development, maintenance
• Impact on children
• Reusability

Response for a Class (RFC)

• Number of methods potentially executed in a response to a message

RFC = number of methods + number of distinct methods invoked

• Complexity
• Testing effort

Lack of Cohesion of Methods (LCOM)

• Degree of cohesion and methods in a class

  • Ik set of instance variables used by method k
  • P = {(Ik, Ij)} such that Ik and Ij do not intersect
  • Q = {(Ik, Ij)} such that Ik and Ij do intersect
  • if |P| > |Q| then LCOM = |P| - |Q|
  • else LCOM = 0

• How well methods are related

  • Internal cohesion
  • LCOM > 0 means lack of cohesion

Coupling Between Object Classes (CBO)

Degree of Class Coupling

• Uses (C) = |distinct classes that use C|
• Used-by(C) = |distinct classes that C use|
• CBO(C) = Uses(C) + Used-by(C)

Depth of Inheritance Tree (DIT)

• max length from root to a class
  • Complexity
  • understandability
  • reuse

Number of children

• number immediate subclasses as a class
  • reuse
  • misuse of sub-classing
  • testing