High Cohesion

Principle

Ensure that each class is highly cohesive.

Object-Oriented designers use the word cohesion to describe everything in a class is related to its central purpose. It is a measure for how well the internal parts of a class (e.g. the methods and attributes) belong together.

info

A highly cohesive class is one that only comprises responsibilities which belong together. A class ideally has a single responsibility.

Why does this matter?

  • Applications that are easy to change consist of classes that are easy to reuse.
  • A class that has many responsibilities is difficult to reuse.
  • A class that has several responsibilities, has many reasons to change. When it changes,
    • it may change for a reason that is unrelated to your use of it.
    • there's a possibility of breaking every class that depends on it.
How to figure out if a class is not cohesive?

Here is one strategy: describe your class in one sentence.

  • if you cannot, it probably has too many responsibilities.
  • if your description has "and"/"or" in it, it probably has too many responsibilities.
Take home message

A class should do the smallest possible useful thing.

Example Design

The examples are (partial) designs for the MyBook App.

You may have been tempted to move author-related information into the Book class since an author has only three data fields, producing the Book class below:

But now the Book class (arguably) has too many responsibilities. It would have to change if we change, e.g. author information.

On the other hand, separating Publisher and Book may be an overkill (as far as we are concernd in our little app), since publisher is just a name (i.e. String) that may very well be integrated into (and seen as part of) Book. So, the design below is trying too much to increase cohesion.

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