Cohesion, Coupling, and Separation of Concerns

There are three important ideas in organizing your code into classes and methods:

Cohesion of code is how focused a portion of code is on a unified purpose. This applies to both individual methods and to a class. The higher the cohesion, the easier it is for you to understand a method or a class. Also a cohesive method is easy to reuse in combination with other cohesive methods. A method that does several things is not useful to you if you only want to do one of the things later.

Separation of concerns allows most things related to a class to take place in the class where they are easy to track, separated from other classes. Data most used by one class should probably reside in that class. Cohesion is related to separation of concerns: The data and methods most used by one class should probably reside in that class, so you do not need to go looking elsewhere for important parts. Also, if you need to change something, where concerns are separated into cohesive classes, with the data and logic in one place, it is easier to see what to change, and the changes are likely to be able to be kept internal to the class, affecting only its internal implementation, not its public interface.

Some methods are totally related to the connection between classes, and there may not be a clear candidate for a class to maximize the separation of concerns. One thing to look at is the number of references to different classes. It is likely that the most referred to class is the one where the method should reside.

Coupling is the connections between classes. If there were no connections to a class, no public interface, it would be useless except all by itself. The must be some coupling between classes, where one class uses another, but with increased cohesion and strong separation of concerns you are likely to be able to have looser coupling. Limiting coupling makes it easier to follow your code. There is less jumping around. More important, it is easier to modify the code. There will be less interfacing between classes, so if you need to change the public interface of a class, there are fewer places in other classes that need to be changed to keep in sync.

Aim for strong cohesion, clear separation of concerns, and loose coupling. Together they make your code clearer, easier to modify, and easier to debug.

To see a fairly large, multi-class project, designed to follow the ideas in this section, see the zuul/src subfolder of You can put it in a project and it starts frpm class Game. The use of the interface, Response, there has a lot to do with the ideas in this section.