CKJM Chidamber and Kemerer Java Metrics

Designed for use with CKJM and Ant.

WMC - Weighted methods per class

A class's weighted methods per class WMC metric is simply the sum of the complexities of its methods. As a measure of complexity we can use the cyclomatic complexity, or we can abritrarily assign a complexity value of 1 to each method. The ckjm program assigns a complexity value of 1 to each method, and therefore the value of the WMC is equal to the number of methods in the class.

DIT - Depth of Inheritance Tree

The depth of inheritance tree (DIT) metric provides for each class a measure of the inheritance levels from the object hierarchy top. In Java where all classes inherit Object the minimum value of DIT is 1.

NOC - Number of Children

A class's number of children (NOC) metric simply measures the number of immediate descendants of the class.

CBO - Coupling between object classes

The coupling between object classes (CBO) metric represents the number of classes coupled to a given class. This coupling can occur through method calls, field accesses, inheritance, arguments, return types, and exceptions.

RFC - Response for a Class

The metric called the response for a class (RFC) measures the number of different methods that can be executed when an object of that class receives a message (when a method is invoked for that object). Ideally, we would want to find for each method of the class, the methods that class will call, and repeat this for each called method, calculating what is called the transitive closure of the method's call graph. This process can however be both expensive and quite inaccurate. In ckjm, we calculate a rough approximation to the response set by simply inspecting method calls within the class's method bodies. This simplification was also used in the 1994 Chidamber and Kemerer description of the metrics.

LCOM - Lack of cohesion in methods

A class's lack of cohesion in methods (LCOM) metric counts the sets of methods in a class that are not related through the sharing of some of the class's fields. The original definition of this metric (which is the one used in ckjm) considers all pairs of a class's methods. In some of these pairs both methods access at least one common field of the class, while in other pairs the two methods to not share any common field accesses. The lack of cohesion in methods is then calculated by subtracting from the number of method pairs that don't share a field access the number of method pairs that do. Note that subsequent definitions of this metric used as a measurement basis the number of disjoint graph components of the class's methods. Others modified the definition of connectedness to include calls between the methods of the class. The program ckjm follows the original (1994) definition by Chidamber and Kemerer.

Ca - Afferent couplings

A class's afferent couplings is a measure of how many other classes use the specific class. Ca is calculated using the same definition as that used for calculating CBO.

NPM - Number of Public Methods

The NPM metric simply counts all the methods in a class that are declared as public. It can be used to measure the size of an API provided by a package.