Coupling and Cohesion: The Two Cornerstones of OO Programming
Object-oriented programming has two main objectives: to build highly cohesive classes and to maintain loose coupling between those classes. High-cohesion means well-structured classes and loose coupling means more flexible, extensible software. Applying object-oriented metrics to your design and code can help you determine whether you’ve achieved these goals.
What is Cohesion?
In OO methodology, classes contain certain data and exhibit certain behaviours. This concept may seem fairly obvious, but in practice, creating well-defined and cohesive classes can be tricky. Cohesive means that a certain class performs a set of closely related actions. A lack of cohesion, on the other hand, means that a class is performing several unrelated tasks. Though lack of cohesion may never have an impact on the overall functionality of a particular class—or of the application itself—the application software will eventually become unmanageable as more and more behaviours become scattered and end up in wrong places.
Thus, one of the main goals of OO design is to come up with classes that are highly cohesive. Luckily, there’s a metric to help youverify that you’ve designed a cohesive class.
The LCOM Metric: Lack of Cohesion in Methods
The Lack of Cohesion in Methods metric is available in the following three formats:
LCOM1: Take each pair of methods in the class and determine the set of fields they each access. If they have disjointed sets of field accesses, the count P increases by one. If they share at least one field access, Q increases by one. After considering each pair of methods:RESULT = (P > Q) ? (P - Q) : 0
A low value indicates high coupling between methods. This also indicates potentially high reusability and good class design. Chidamber and Kemerer provided the definition of this metric in 1993.
LCOM2: This is an improved version of LCOM1. Say you define the following items in a class:m: number of methods in a classa: number of attributes in a class.mA: number of methods that access the attribute a.sum(mA): sum of all mA over all the attributes in the class.LCOM2 = 1- sum(mA)/(m*a)
If the number of methods or variables in a class is zero (0), LCOM2 is undefined as displayed as zero (0).
LCOM3: This is another improvement on LCOM1 and LCOM2 and is proposed by Henderson-Sellers. It is defined as follows:LCOM3 = (m - sum(mA)/a) / (m-1) where m, a, mA, sum(mA) are as defined in LCOM2.
The following points should be noted about LCOM3:
- The LCOM3 value varies between 0 and 2. LCOM3>1 indicates lack of cohesion and is considered a kind of alarm.
- If there is only one method in a class, LCOM 3 is undefined and also if there are no attributes in a class LCOM3 is also undefined and displayed as zero (0).
Each of these different measures of LCOM has a unique way to calculate the value of LCOM.
- An extreme lack of cohesion such as LCOM3>1 indicates that the particular class should be split into two or more classes.
- If all the member attributes of a class are only accessed outside of the class and never accessed within the class, LCOM3 will show a high-value.
- A slightly high value of LCOM means that you can improve the design by either splitting the classes or re-arranging certain methods within a set of classes.
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