Metrics for Object Oriented Design

Shyam R. Chidamber Chris F. Kemerer

Presented byAmbikadevi Damodaran.

Introduction

Existing OO Design metrics are subject to one or more criticism.There are lot of ongoing research for new suite of metrics for OO design.

Four Major steps involved in Object Oriented Design are1.Identifying Classes.2.Identifying the semantics of classes.3.Identifying relationships between Classes.4.Implementation of classes.

Different Representations of OOD

1.D = (A,R1…Rn, O1…..Om)A is a set of Object-elements,R1..Rn are relations,O1..Om are operations on A.

2.Viewed as collection of things with properties.

Important properties Coupling and Cohesion are derived based on the above representations.

Metric 1. Weighted Methods Per Class(WMC)

Let a class be C1 with methods M1..Mn. Let c1…cn be the complexity of the methods. Then

WMC = Summation of complexity c1…cn

View Points1.The Number of methods and complexity of methods involved is a predicator of how much time and effort is required to develop

and maintain the class.2.The larger the number of methods in a class the greater the potential impact on children, since children will inherit all the

methods defined in the class.3.Classes with large numbers of methods are likely to be more

application specific, limiting the possibility of reuse.

Metric 2. Depth of Inheritance(DIT)

Depth of inheritance of the class is the DIT metric for the class. DIT is a measure of how many ancestor classes can

potentially affect this class.

View points1.The deeper a class is in the hierarchy, the greater the number of methods it is likely to inherit making it more

complex.2.Deeper trees constitute greater design complexity, since

more methods and classes are involved.3.The deeper a particular class is in the hierarchy, the

greater the potential reuse of inherited methods.

Metric 3. Number of Children

It is a measure of how many subclasses are going to inherit the methods of the parent class.

Viewpoints1.Greater the number of children, greater the reuse, since the

inheritance is a form of reuse.2.Greater the number of children, the greater the likelihood of improper abstraction of the parent class. If a class has a large number of children, it may be a case of misuse of sub classing.

3. The number of children gives an idea of the potential influence a class has on the design. If a class has a large

number of children, it may require more testing of the methods in that class.

Metric 4. Coupling between object classes

Objects are coupled if one of them acts on the other.

Viewpoints1.More independent the class is, the easier it is

to reuse.2.The larger the number of couples, the

maintenance is more difficult.3.The higher the inter-object class coupling the

testing is more rigorous.

Metric 5. Response for a class(RFC)

The response set of a class is a set of methods that can potentially be executed in response to a message received

by an object of that class.

View points1.With more number of methods, testing and debugging is

more complicated.2.The larger the number of methods that can be invoked

from a class, the greater the complexity of the class.

Metric 6 Lack of Cohesion in Methods(LCOM)

Lack of cohesion means, classes are not properly encapsulated, so it means the classes can be

split into one or more subclasses.

View Points1.Low cohesion means increased complexity.

2.Low cohesion also increases the likelihood of errors.

Conclusion

1.These metrics reflects the viewpoints of OO developers.

2.These metrics can be used as a vehicle to address consistency of entire application.3.By applying metrics, the areas that needs more testing can be identified.4.Any one of the metrics can be chosen depending on the needs of the organization.