CS5228 Principles of Software Design and

Software Engineeringg g

D i M t iDesign Metrics

Architectural Analysis/Design ConcernsArchitecturalAnalysis/DesignConcerns

Involvessystemlevel,largescaleandbroadproblemswhoseresolutionusuallyinvolveslargep y gscaleorfundamentaldesigndecisions

Involves considering interdependencies and Involvesconsideringinterdependenciesandtradeoffs.

Generationandevaluationofalternativesolutions.solutions.

MeasuringDesignQualityg g y Why?

One specification many Designs!OnespecificationmanyDesigns! Subjective/Objective

h d l i k i Researchersdevelopingkeymetrics Examplep

MetricforCoupling System ComplexitySystemComplexityComplexity=StructuralComplexity

+ Data Complexity+DataComplexity

Measuring Design Quality (cont )MeasuringDesignQuality(cont.)

Att ib t P i it ( t) D i 1 D i 2 D i 3

ComparisonTables

Attribute Priority(wt) Design1 Design2 Design3

Reuse

ExtendibilityExtendibility

Complexity

Design Review QsDesignReviewQs1. Isthisdesignasolutiontotheproblem?g p

2. Isthedesignmodular,wellstructured?

3 I h d i d d bl3. Isthedesignunderstandable

4. IsitPortable?

5. Isitreusable?

6. Isiteasytomodifyandextend?

7. Does it support ease of testing ?7. Doesitsupporteaseoftesting?

Design Review QsDesignReviewQs8. Doesthedesignmaximizeperformancewhere

appropriate?9. Doesthedesignreusepatterns/code?10.Arethealgorithmsappropriatecantheybe

improved?p11.Isthedesignwelldocumented?12 Does the design cross reference the components12.Doesthedesigncrossreferencethecomponents

anddatawiththerequirements?13 D th d i t h i f f lt13.Doesthedesignusetechniquesforfault

tolerance?

Design MetricsDesignMetrics

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

A large number metrics have been proposed for OO designs

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

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have received little or no validation.

ExampleDesignMetricsp g[Chidamber &Kemerer]

1. DesignComplexityW i ht d M th d P Cl WeightedMethodsPerClass

Sumofmethodcomplexities

2. DepthofInheritance Maximum Length of the path in the hierarchyMaximumLengthofthepathinthehierarchy

fromtheclasstotherootoftheinheritancetree

3 Number of Children3. NumberofChildren Numberofimmediatesubclasses

Example Design Metrics (Cont )ExampleDesignMetrics(Cont.)

4. CouplingC t f i ti ith th l Countofassociationswithotherclasses

5. LackofCohesioninMethods

Other Example Design MetricsOtherExampleDesignMetrics

1. NumberofKeyClasses2 Number of Support Classes2. NumberofSupportClasses3. AveragenumberofKeyClassespersupport

lclass4. Classsize5. Numberofoperationsoverriddenbya

subclass (NOO).subclass(NOO).6. Numberofoperationsaddedbyasubclass

Other Example Design Metrics (cont )OtherExampleDesignMetrics(cont.)

7. SpecializationIndex(Lorenz,Kidd)SI NOO l l / t t l Cl M th dSI=NOOxlevel/totalClassMethods

8. AveragenumberofParametersperoperation

9. Percentofpublic/protected

10 A t f bli t d t b10.Amountofpublicaccesstodatamembers

11.Fan_in (numberofclassescalling)_ ( g)

12.Fan_out (numberofclassescalledby)

Weighted Methods Per Class (WMC)Weighted Methods Per Class (WMC) Assumption: The effort in developing a class isAssumption:Theeffortindevelopingaclassisdeterminedbythenumberofmethods.

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

n cWMCicLet denote the complexity of method iM 1i i

cWMCi i

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

Depth of inheritance tree (DIT)Depthofinheritancetree(DIT)

Inheritance increases coupling. Changing classes becomes harder.D h f I h i (DIT) f l C i h l h f h 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.

DIT is significant in predicting error proneness of a classclass.

Hi h DIT l d t hi h13

Higher DIT leads to higher error-proneness.

Coupling between classes (CBC)Couplingbetweenclasses(CBC)

Class C1 is coupled to class C2 if at least one method of C1 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.

Most classes are self contained and have CBC=0.

Interface classes tend to have higher CBC values. CBC is

i ifi t i di ti f lt f lsignificant in predicting fault-proneness of classes.

Coupling Factor (CF)CouplingFactor(CF)

is_client(x,y)=1iff arelationshipexistsb t th li t l d th lbetweentheclientclassandtheserverclass.0otherwise.

TCisthetotalnumberofrelationshipspossiblepossible

CFis[0,1]with1meaninghighcoupling

Numberofchildren(NOC)( ) NOCisthenumberofimmediatesubclassesofC.

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

L NOC l i i t d ith l b bilit Larger NOC value is associated with lower probability of detecting faults in that class.

Lackofcohesioninmethods(LCOM)

LetI1andI2denotesetsofinstancevariablesaccessedbyh d d l lmethodsM1andM2,respectively,inclassC.

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.Q p

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

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

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

NumberofOperationsOverridden(NOO) AlargenumberforNOOindicatespossibleproblemswiththedesignp g

Poorabstractionininheritancehierarchy

NumberofOperationsAdded(NOA) Thenumberofoperationsaddedbyasubclass As operations are added it is farther away from Asoperationsareaddeditisfartherawayfromsuperclass

A d th i NOA h ld d AsdepthincreasesNOAshoulddecrease

SpecializationIndex(SI) SI=[NOO*L]/M total[ ] / _ ListhelevelinclasshierarchyM total is the total number of methods M_total isthetotalnumberofmethods

Highervaluesindicateclassinhierarchythatdoesf h b inotconformtotheabstraction

MethodInheritanceFactor(MIF)

Mi(Ci) is the number of methods inherited andMi(Ci)isthenumberofmethodsinheritedandnotoverriddeninCi

( ) i h b f h d h b Ma(Ci)isthenumberofmethodsthatcanbeinvokedwithCi

MIFis[0,1] MIF near 1 means little specializationMIFnear1meanslittlespecializationMIFnear0meanslargechange