A Validation of Object-Oriented Design Metrics As Quality Indicators Basili et al. IEEE TSE Vol. 22, No. 10, Oct. 96

Abstract Suite of OOD metrics by Chidamber. Predictors of fault-prone classes? Can be used as early quality indicators? Other study: frequence of maintenance changes. 8 medium sized information management systems

Metric CBO: coupling between object classes: a class is coupled to another one if it uses its member functions and/or instance variables. CBO = number of classes to which a given class is coupled.

Connection to Law of Demeter Following LoD or reducing the number of violations reduces CBO because LoD only allows good coupling.

Law of Demeter: Violation class A {public: void m(); P p(); B b; }; class B {public: C c; }; class C {public: void foo(); }; class P {public: Q q(); }; class Q {public: void bar(); }; void A::m() { this.b.c.foo(); this.p().q().bar();} CBO(A) = 4 With LoD: CBO(A) = 2 CBO(A)= {B,C,P,Q}

Metric RFC: response for a class. This is the number of methods that can potentially be executed in response to a message received by an object of that class.

Connection to Law of Demeter Following LoD or reducing the number of violations reduces RFC because LoD allows fewer methods to be called.

Law of Demeter: Violation class A {public: void m(); P p(); B b; }; class B {public: C c; }; class C {public: X foo(); }; class P {public: Q q(); }; class Q {public: Y bar(); }; void A::m() { this.b.c.foo(); this.p().q().bar();} RFC(A) = 6 With LoD: RFC(A) = 4 RFC(A)= {b,c,p,q,foo,bar} Assume data member access through get method RFC(A)= {b,p,foo2,bar2} But …

Law of Demeter: Violation class A {public: void m(); P p(); B b; }; class B {public: C c; void foo2() {c.foo();}; class C {public: X foo(); }; class P {public: Q q(); void bar2(){q().bar(); }; class Q {public: Y bar(); }; void A::m() { this.b.foo2(); this.p().bar2();} RFC(A) = 4 RFC(B) = 2 RFC(P) = 2 8 With LoD: Assume data member access through get method Without LoD: = 6 LoD spreads RFC but it might increase it!

Law of Demeter: Violation Illustrate better separation class A {public: void m(); P p(); B b; }; class B {public: C c; }; class C {public: X foo(); }; class P {public: Q q(); }; class Q {public: Y bar(); }; void A::m() { X x=M.cg.fetch(this,”from A to X”); Y y=M.cg.fetch(this,”from A to Y”);} With LoD: RFC(A) = 1 RFC(A)={fetch} Assume data member access through get method RFC(A) = 4 RFC(B) = 2 RFC(P) = 2 8 CBO(A)= {DJ.ClassGraph}

Class Diagram A BC b c PQ p() q() foo() bar() m X Y from A to X from A to Y

Hypotheses H-CBO: Highly coupled classes are more fault-prone than weakly coupled classes because they depend more heavily on methods and objects defined elsewhere. H-RFC: Classes with larger response sets implement more complex functionalities and are, therefore, more fault-prone.

RFC (on projects) X axis: values of metric. Y axis: number of classes. Y X Probability of fault detection increases

CBO (on projects) X axis: values of metric. Y axis: number of classes. Y X Probability of fault detection increases

Results RFC is very significant. The H-RFC hypothesis is supported. CBO is significant, particularly for UI classes.

Analysis Methodology Response variable: binary: was a fault detected in a class during testing? Used logistic regression, a standard technique used in experimental sciences. Alternatives would be: classification trees, optimized set reduction, or neural networks.

Study Participants Four months Students in a class on OO software analysis and design Control differences in skills: randomly grouped into eight teams. Used blocking: eight most experienced students randomly assigned to a different group.

Development Process for Video Store Application Sequential software engineering life-cycle model based on waterfall model: analysis, design, implementation, testing and repair. Requirement and design document were checked. OMT (Rumbaugh) was used C++ with MotifApp, GNU library, database library.