CSC/ECE 517 Fall 2009/wiki3 3 cp

Common Closure Principle

In simple words the Common Closure Principle refers to classes that change topether, belong together, this is a very simple definition taken from the book Java Design: Objects, UML and Process written by Kirk Knoernschild.

A more explained definition of Common Closure Principle, given by Robert Martin, is

      THE CLASSES IN A PACKAGE SHOULD BE CLOSED TOGETHER AGAINTS THE SAME 
      KINDS OF CHANGES. A CHANGE THAT AFFECTS A PACKAGE AFFECTS ALL THE 
      CLASSES WITHIN THAT PACKAGE. 
      

See Granularity PDF Article written by Robert Martin for more details

In other words, what affects one, affects all broader and detailed explanations are given in this wikiwork.

Introduction and Objective

There are several web pages that explain in few words what is Common Closure Principle, but they fall short by just giving definitions and very few examples.

Therefore the objective of the wiki work is to dedicate a deeper investigation on this topic with examples.

A deep definition of Common Closure Principle CCP

CCP is another package design principle which tries to look at maintainability rather than usability. The principle advises on which classes should be packaged together from a change and distribution point of view

Even though most of the class design principles advocate loose coupling between classes, so that they can change without affecting the other, dependencies between classes cannot be avoided. CCP tries to look at classes that are dependent on each other from a code change perspective. Even though Open-Closed Principle (OCP) recomments full closure to code change, it is not always possible. Sometimes some of the class design principles are violated consciously to improve performance, just like denormalization in databases. However this violation should be a conscious decision and the product should be ready for changes because of it.

To enable easy distribution/update/release and maintainability, it is advisable to localise the changes to a package. Only the modified package can then be released, making the update easier. Even if the changes cannot be restricted to a single package, minimal packages should be required to be changed. To get an idea of the code change dependencies, we can take guidance from Law Of Demeter (LOD) principle, which tries to point out classes which know each other’s structures.

One issue is when dependencies between packages in a design should be in the direction of stability of the packages, in other words a package should only depend upon packages that are more stable than it is.

Some volatility is necessary if the design is to be maintained. This is achieved by using the Common Closure Principle, in this way we design packages to be volatile and we expect them to change. Any package that we expect to be volatile should not be depended upon by a package that is difficult to change.

Some things we don’t want to change. For example architectural decisions should be stable and not at all volatile. Therefore classes that encapsulate the high level design should be stable.

External Links

Bob Martin's

Appendix

Open/Closed Principle: Software entities (classes, modules, etc) should be open for extension, but closed for modification.

Liskov Substitution Principle: Derived classes must be usable through the base class interface without the need for the user to know the difference.

Dependency Inversion Principle: Details should depend upon abstractions instead of abstractions depend upon details.

Interface Segregation Principle: Many client specific interfaces are better than one general purpose interface.

Reuse/Release Equivalency Principle: They are almost the same, the only components that are released through a tracking system can be effectively reused.

Common Closure Principle: Classes that change together, belong together.

Common Reuse Principle: Classes that are not reused together should not be grouped together.

Acyclic Dependencies Principle: The dependency structure for released components must be a directed acyclic graph.

The Stable Dependencies Principle: Dependencies between released categories must run in the direction of stability. The dependee must be more stable than the depender.

The Stable Abstractions Principle: A completely stable category should consist of nothing but abstract classes