Design MetricsSoftware Engineering

Fall 2003

Aditya P. Mathur

Last update: October 28, 2003

October 23, 2001 Design Metrics 2

Design Metrics

Design Metrics are useful in measuring the complexity and “goodness” of a design.

A large number metrics have been proposed for OO designs.

Some of these have been validated experimentally, others are mere proposals or have received little or no validation.

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Effort

Assumption: The effort in developing a class is determined by the number of methods.

Hence the overall complexity of a class can be measured as a function of the complexity of its methods.

Proposal: Weighted Methods per class (WMC)

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WMC

Let class C have methods M1, M2, .....Mn.

icLet denote the complexity of method

iM

∑=

=n

1ii

cWMC

How to measure WMC?

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WMC: validation

Most classes tend to have a small number of methods, are simple, and provide some specific abstraction and operations.

WMC metric has a reasonable correlation with fault-proneness of a class.

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Depth of inheritance tree (DIT)

Depth of a class in a class hierarchy determines potential for re-use as more methods are available. Deeper classes have higher potential for re-use though are more complex.

Inheritance increases coupling. Changing classes becomes harder.

Depth of Inheritance (DIT) of class C is the length of the shortest path from the root of the inheritance tree to C.

In the case of multiple inheritance DIT is the maximum length of the path from the root to C.

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DIT evaluation

Basili et al. study,1995. Chidamber and Kemerer study, 1994.

• Most classes tend to be close to the root.

• Maximum DIT value found to be 10.

• Most classes have DIT=0.

• DIT is significant in predicting error proneness of a class. Higher DIT leads to higher error-proneness.

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Number of children (NOC)

NOC is the number of immediate subclasses of C.

Higher values of NOC suggest reuse of the definitions in the super-class in a larger number of subclasses.

Higher NOC suggests the extent of influence of a class on other elements of a design. Higher influence demands higher quality of that class.

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Validation of NOC

Classes generally have a small NOC value.

Vast majority have NOC=0.

Larger NOC value is associated with lower probability of detecting faults in that class.

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Coupling between classes (CBC)

Class C1 is coupled to class C2 if at least one method of C1 uses a method or an instance variable of C2.

CBC of C=total number of other classes to which C is coupled.

Coupling is usually easy to identify though often pointers may make it difficult.

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Validation of CBC

Most classes are self contained and have CBC=0.

CBC is significant in predicting fault-proneness of classes.

Interface classes tend to have higher CBC values.

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Response for a class (RFC)

Response set of class C is the total number of methods that can be invoked when a message is sent to an object of C.

This includes all methods of C and any methods executed outside of C as a result of this message.

RFC of class C is the cardinality of the response set of C.

Note that even when CBC=1 RFC may be high. This indicates that the “volume” of interaction is high.

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Validation of RFC

Most classes tend to invoke a small number of methods (low RFC values).

Classes for interface objects tend to have larger RFC values.

RFC is very significant in predicting the fault-proneness of a class.

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Lack of cohesion in methods (LCOM) [1]

Let I1 and I2 denote sets of instance variables accessed by methods M1 and M2, respectively, in class C.

M1 and M2 are considered similar, or cohesive, if I1 and I2 are not disjoint.

Let Q be the set of all cohesive method pairs.

Let P be the set of all non-cohesive method pairs.

LCOM=|P| - |Q| if |P| > |Q|, 0 otherwise.

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LCOM [2]

A larger number of cohesive pairs implies smaller LCOM.

A high value of LCOM suggests that a class is trying to support multiple abstractions. Perhaps the class needs to be partitioned into smaller and more cohesive classes.

LCOM is not found to be very significant in predicting fault-proneness.

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Guidelines for interpretation [1]

Ref: http://satc.gsfc.nasa.gov/support/STC_APR98/apply_oo/apply_oo.html

METRIC OBJECTIVE

Cyclomatic Complexity Low

Lines of Code/Executable Statements Low

Comment Percentage ~ 20 – 30 %

Weighted Methods per Class Low

Response for a Class Low

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Guidelines for interpretation [2]

METRIC OBJECTIVE

Lack of Cohesion of Methods Cohesion of Methods/ Low/ High

Coupling Between Objects Low

Depth of Inheritance Low (trade-off)

Number of Children Low (trade-off)

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Guidelines for interpretation [3]

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Guidelines for interpretation [4]

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Guidelines for interpretation [5]

Almost 66% of this project’s classes are below other classes in the tree, which indicates a moderate level of reuse.

Higher percentages for DIT’s of 2 and 3 would show a higher degree of reuse, but increased complexity.

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Summary of OO metrics

SOURCE METRIC OO CONSTRUCT

Traditional Cyclomatic complexity (CC) MethodTraditional Lines of Code (LOC) MethodTraditional Comment percentage (CP) Method

NEW Weighted methods per class Class/MethodNEW Response for a class (RFC) Class/MessageNEW Lack of cohesion of methods (LCOM) Class/CohesionNEW Coupling between objects (CBC) CouplingNEW Depth of inheritance tree (DIT) InheritanceNEW Number of children (NOC) Inheritance

Ref: http://satc.gsfc.nasa.gov/support/STC_APR98/apply_oo/apply_oo.html