| The idea of cohesion was introduced in a paper by Wayne Stevens, Glenford Myers, and Larry Constantine (1974). Other more modern concepts, including abstraction and encapsulation, tend to yield more insight at the class level (and have, in fact, largely superceded cohesion at the class level), but cohesion is still alive and well as the workhorse design heuristic at the individual-routine level. For routines, cohesion refers to how closely the operations in a routine are related. Some programmers prefer the term "strength": how strongly related are the operations in a routine? A function like Cosine() is perfectly cohesive because the whole routine is dedicated to performing one function. A function like CosineAndTan() has lower cohesion because it tries to do more than one thing. The goal is to have each routine do one thing well and not do anything else. Cross-Reference For a discussion of cohesion in general, see "Aim for Strong Cohesion" in Section 5.3.
 | The payoff is higher reliability. One study of 450 routines found that 50 percent of the highly cohesive routines were fault free, whereas only 18 percent of routines with low cohesion were fault free (Card, Church, and Agresti 1986). Another study of a different 450 routines (which is just an unusual coincidence) found that routines with the highest coupling-to-cohesion ratios had 7 times as many errors as those with the lowest coupling-to-cohesion ratios and were 20 times as costly to fix (Selby and Basili 1991). |
Discussions about cohesion typically refer to several levels of cohesion. Understanding the concepts is more important than remembering specific terms. Use the concepts as aids in thinking about how to make routines as cohesive as possible. Functional cohesion is the strongest and best kind of cohesion, occurring when a routine performs one and only one operation. Examples of highly cohesive routines include sin(), GetCustomerName(), EraseFile(), CalculateLoanPayment(), and AgeFromBirthdate(). Of course, this evaluation of their cohesion assumes that the routines do what their names say they do if they do anything else, they are less cohesive and poorly named. Several other kinds of cohesion are normally considered to be less than ideal: Sequential cohesion exists when a routine contains operations that must be performed in a specific order, that share data from step to step, and that don't make up a complete function when done together. An example of sequential cohesion is a routine that, given a birth date, calculates an employee's age and time to retirement. If the routine calculates the age and then uses that result to calculate the employee's time to retirement, it has sequential cohesion. If the routine calculates the age and then calculates the time to retirement in a completely separate computation that happens to use the same birth-date data, it has only communicational cohesion. How would you make the routine functionally cohesive? You'd create separate routines to compute an employee's age given a birth date and compute time to retirement given a birth date. The time-to-retirement routine could call the age routine. They'd both have functional cohesion. Other routines could call either routine or both routines. Communicational cohesion occurs when operations in a routine make use of the same data and aren't related in any other way. If a routine prints a summary report and then reinitializes the summary data passed into it, the routine has communicational cohesion: the two operations are related only by the fact that they use the same data. To give this routine better cohesion, the summary data should be reinitialized close to where it's created, which shouldn't be in the report-printing routine. Split the operations into individual routines. The first prints the report. The second reinitializes the data, close to the code that creates or modifies the data. Call both routines from the higher-level routine that originally called the communicationally cohesive routine. Temporal cohesion occurs when operations are combined into a routine because they are all done at the same time. Typical examples would be Startup(), CompleteNewEmployee(), and Shutdown(). Some programmers consider temporal cohesion to be unacceptable because it's sometimes associated with bad programming practices such as having a hodgepodge of code in a Startup() routine. To avoid this problem, think of temporal routines as organizers of other events. The Startup() routine, for example, might read a configuration file, initialize a scratch file, set up a memory manager, and show an initial screen. To make it most effective, have the temporally cohesive routine call other routines to perform specific activities rather than performing the operations directly itself. That way, it will be clear that the point of the routine is to orchestrate activities rather than to do them directly. This example raises the issue of choosing a name that describes the routine at the right level of abstraction. You could decide to name the routine ReadConfig-FileInitScratchFileEtc(), which would imply that the routine had only coincidental cohesion. If you name it Startup(), however, it would be clear that it had a single purpose and clear that it had functional cohesion.
The remaining kinds of cohesion are generally unacceptable. They result in code that's poorly organized, hard to debug, and hard to modify. If a routine has bad cohesion, it's better to put effort into a rewrite to have better cohesion than investing in a pinpoint diagnosis of the problem. Knowing what to avoid can be useful, however, so here are the unacceptable kinds of cohesion: Procedural cohesion occurs when operations in a routine are done in a specified order. An example is a routine that gets an employee name, then an address, and then a phone number. The order of these operations is important only because it matches the order in which the user is asked for the data on the input screen. Another routine gets the rest of the employee data. The routine has procedural cohesion because it puts a set of operations in a specified order and the operations don't need to be combined for any other reason. To achieve better cohesion, put the separate operations into their own routines. Make sure that the calling routine has a single, complete job: GetEmployee() rather than GetFirstPartOfEmployeeData(). You'll probably need to modify the routines that get the rest of the data too. It's common to modify two or more original routines before you achieve functional cohesion in any of them. Logical cohesion occurs when several operations are stuffed into the same routine and one of the operations is selected by a control flag that's passed in. It's called logical cohesion because the control flow or "logic" of the routine is the only thing that ties the operations together they're all in a big if statement or case statement together. It isn't because the operations are logically related in any other sense. Considering that the defining attribute of logical cohesion is that the operations are unrelated, a better name might "illogical cohesion." One example would be an InputAll() routine that inputs customer names, employee timecard information, or inventory data depending on a flag passed to the routine. Other examples would be ComputeAll(), EditAll(), PrintAll(), and SaveAll(). The main problem with such routines is that you shouldn't need to pass in a flag to control another routine's processing. Instead of having a routine that does one of three distinct operations, depending on a flag passed to it, it's cleaner to have three routines, each of which does one distinct operation. If the operations use some of the same code or share data, the code should be moved into a lower-level routine and the routines should be packaged into a class. It's usually all right, however, to create a logically cohesive routine if its code consists solely of a series of if or case statements and calls to other routines. In such a case, if the routine's only function is to dispatch commands and it doesn't do any of the processing itself, that's usually a good design. The technical term for this kind of routine is "event handler." An event handler is often used in interactive environments such as the Apple Macintosh, Microsoft Windows, and other GUI environments. Cross-Reference Although the routine might have better cohesion, a higher-level design issue is whether the system should be using a case statement instead of polymorphism. For more on this issue, see "Replace conditionals with polymorphism (especially repeated case statements)" in Section 24.3
Coincidental cohesion occurs when the operations in a routine have no discernible relationship to each other. Other good names are "no cohesion" or "chaotic cohesion." The low-quality C++ routine at the beginning of this chapter had coincidental cohesion. It's hard to convert coincidental cohesion to any better kind of cohesion you usually need to do a deeper redesign and reimplementation.
 | None of these terms are magical or sacred. Learn the ideas rather than the terminology. It's nearly always possible to write routines with functional cohesion, so focus your attention on functional cohesion for maximum benefit. |
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