Design Metrics Software Engineering Fall 2003 Aditya P. Mathur Last update: October 28, 2003

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. October 23, 2001 Design Metrics

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) October 23, 2001 Design Metrics

WMC Let denote the complexity of method Let class C have methods M1, M2, .....Mn. Let denote the complexity of method How to measure WMC? October 23, 2001 Design Metrics

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. October 23, 2001 Design Metrics

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. October 23, 2001 Design Metrics

DIT evaluation Most classes tend to be close to the root. 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. October 23, 2001 Design Metrics

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. October 23, 2001 Design Metrics

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. October 23, 2001 Design Metrics

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. Coupling is usually easy to identify though often pointers may make it difficult. CBC of C=total number of other classes to which C is coupled. October 23, 2001 Design Metrics

Validation of CBC Most classes are self contained and have CBC=0. Interface classes tend to have higher CBC values. CBC is significant in predicting fault-proneness of classes. October 23, 2001 Design Metrics

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. October 23, 2001 Design Metrics

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. October 23, 2001 Design Metrics

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. October 23, 2001 Design Metrics

October 23, 2001 Design Metrics

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. October 23, 2001 Design Metrics

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 October 23, 2001 Design Metrics

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) October 23, 2001 Design Metrics

Guidelines for interpretation [3] October 23, 2001 Design Metrics

Guidelines for interpretation [4] October 23, 2001 Design Metrics

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. October 23, 2001 Design Metrics

Summary of OO metrics SOURCE METRIC OO CONSTRUCT Ref: http://satc.gsfc.nasa.gov/support/STC_APR98/apply_oo/apply_oo.html SOURCE METRIC OO CONSTRUCT Traditional Cyclomatic complexity (CC) Method Traditional Lines of Code (LOC) Method Traditional Comment percentage (CP) Method NEW Weighted methods per class Class/Method NEW Response for a class (RFC) Class/Message NEW Lack of cohesion of methods (LCOM) Class/Cohesion NEW Coupling between objects (CBC) Coupling NEW Depth of inheritance tree (DIT) Inheritance NEW Number of children (NOC) Inheritance October 23, 2001 Design Metrics