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Cohesion is the degree to which the methods of a single class are tight tied together. When two methods in a class do not use a common attribute or a common method, it means that they share nothing and should probably not belong to the same class to respect the Single Responsibility Principle. In other words you can split your class into several new classes to gain modularity at class level.
Sonar computes the The LCOM4 (Lack of Cohesion of Methods) metric computation is based on Hitz & Montazeri. Here is a good introduction: http://www.aivosto.com/project/help/pm-oo-cohesion.html#LCOM4.
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How to Hunt for Bad LCOM4?
Add the LCOM4 widget (was Chidamber & Kemerer widget for Sonar versions prior to 3.3) on your dashboard:
Drill down:
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