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[Software metrics] Lack of Cohesion of Methods - LCOM4

In one of my latest post we talk about Mean Time Between Failure software metric (MTBF). We saw that this metrics is very useful when we need to make an idea about what is the quality of our software that is in production.
Today we will talk about LCOM4 (Lack of Cohesion of Methods). The main information that we receive from this metric is the number of responsibilities (functions) that a class has.
When we have a low cohesion means that we have a bad design, with classes that are very complex. This can also indicates that the class are complex, very big and can produce a lot of bugs.
When we have a high cohesion that we have a very good encapsulation, each class do only one thing but it does very good. Normally, this class are not very long, the complexity of them is not very high and are simple to understand and work with. The only problem that can happen with this class is related to testing. Sometimes, this class are pretty hard to test because the coupling level between them is pretty high.
The number 4 from the name comes from the ‘version’ of this metric. This is not the first time when someone try to measure the cohesion of a class. On the internet there are different formulas. The LCOM for is the one that is recommended in this moment – it seems to be the accurate. It was discover by Hitz and Montazeri and measure the number of connected components in a class. A connected component is a set of method that are related at class level (calls one another and so on). In theory we should have only one component in each class – one functionality.
When we have more than one component in class, that we should split the class in smaller classes. From this perspective two method are related when they use the same fields/variables or when one method calls another method.
In the above example we have 3 different responsibilities in the same class. A depend on B and C, D depend on E and F use G and H.
The ideal value of LCOM4 for a class is one. This indicates us that this class has only one functionality – cohesive class. Any value higher that 1 indicates that the class has to many responsibilities and we should split that class.
Be aware when you have classes with LCOM4 value equal to 0. This is happening when we don’t have any method in a class.
There are cases when we will have classes in our application when LCOM4 value will be higher than 1 and we will not be able to do nothing to it. This can happens when we have a controller from MVC, a view model from MVVM or a user controller for a desktop application. In this cases, when you split your controller based on different functionalities, there are times when there is no need to split the controllers in smaller part only for LCOM4 value.
As I already sad in other posts, LCOM4 like other metrics give us information related to code. It gives us hints where we could have some problems, but we need also to analyze code before making a scope in our live to have this value lowest as possible.
In this post we saw why LOCOM4 is so important in our application. This software metric can give us information related to how many responsibilities a class has. I recommend to try to calculate LCOM4 in your project and see what values you will get.


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