Cohesion - Part 1 What is Cohesion
Cohesion is one the most important yet least understood concepts in OOP. This is the first part of a short series aiming to explain the concept of Cohesion and the benefits you can gain from understanding and correctly applying it.
What is Cohesion
The most basic description of an object in computer science is
“a language mechanism for binding data with methods that operate on that data”
From this we can see that the most fundamental OO design heuristic we can apply is to ensure that an object contains methods which only operate on the data contained within that object; and that the data contained in the object is only operated upon by the methods in that object.
Cohesion is a measure used to determine how closely related an object’s methods are to the data contained within that object. An object is said to exhibit high cohesion when each of its methods uses all the data in that object; an object has low cohesion when its methods uses none of the data. We should aim towards objects exhibiting high cohesion.
We often say that we’d like our code to be loosely-coupled and highly-cohesive. Yet while most programmers understand loose coupling; many do not know what high cohesion means. I don’t really know why this should be, maybe the ordering of the phrase is important and in people’s minds they see loose coupling as being more important and don’t learn about cohesion. If I had my way I’d restate this as “our code should be highly cohesive and where coupling is necessary it should be loose”; in other words by concentrating on high cohesion first you’ll find you need less coupling.
In my mind, cohesion is what really makes OO different to procedural. I’d go as far to say that unless you are continually striving for high cohesion you’re not really doing OO at all. You may be using an OO language but unless your objects display behavior on contained data then your still doing procedural.
How can we Measure Cohesion
There are various methods for calculating the cohesiveness of a class (up to now I’ve written about objects, but in class-based OO languages like Java and C# cohesion is measured on a class rather than an object). The basis of these methods is to calculate the usage of the class’s fields by the class’s methods; the most cohesive a class can be is when all the methods use all of the fields, the least cohesive is when the all the method’s use none of the fields.
The most commonly used calculation method is the Lack of Cohesion of Methods (LCOM) which has the following formula:
1 - (sum(MF) / M * F)
M is the number of methods in the class
F is the number of fields in the class
MF is number of methods which access a given field of the class
The result will be a value in the range 0 - 1, where 0 has the highest cohesion and 1 has the lowest cohesion.
Example LCOM Calculation
If, like me, you’re not a maths gonk, then this can seem slightly daunting so I’ll walk you through an example and all should become clear.
Consider the following class:
So firstly we’ll find the value for M, which is the number of methods in the class. In this case M is five because in addition to standard instance methods we need to include static methods, constructors and property getters/setters (a property with get and set, if we had any, counts as 2). So we have five: constructor, FormattedTotal getter, GetDiscount, FormatTaxAmount and FormatValue.
M = 5
Next, we’ll calculate F. F is the number of fields, so F is three: _customerName, _customerStatus, _total.
F = 3
Then we need to calculate the MF values. To do this we look at each field in turn and count the number methods that access that field, this gives us an MF value for each field thus:
_customerName: MF = 2 (constructor, FormattedTotal)
_customerStatus: MF = 2 (constructor, GetDiscount)
_total: MF = 3 (constructor, FormattedTotal, GetDiscount)
Now we can plug these values into the LCOM formula and calculate to get our final value.
1 - (sum(MF) / M * F)
1 - ((2 + 2 + 3) / 5 * 3)
1 - (7 / 15)
1 - 0.47
So our DiscountCalculator class has a score of 0.53, or maybe put another way 53% of this class is uncohesive (remember we want to get as close to zero as possible). That’s not terrible but I’m sure we can do much better. In the next part I’ll show how to improve the cohesion in this class; and how a focus on cohesion can improve the design of your code.