CSC/ECE 517 Fall 2012/ch2b 2w69 as

The Open/Closed principle

Introduction

In object-oriented programming the Open/Closed principle states<ref name = meyer />,

Software entities (Classes, Modules, Functions, etc.) should be open for extension, but closed for modification.

It sounds like a contradiction in terms, but it's not. All it means is that you should structure an application so that you can add new functionality with minimal modification to existing code. All systems change during their life cycles. One should keep this in mind when designing systems that are expected to last longer than the initial version. What you want to avoid is to have one simple change ripple through the various classes of your application. That makes the system fragile, prone to regression problems, and expensive to extend. To isolate the changes, you want to write classes and methods in such a way that they never need to change once they are written<ref name = microsoft />.

Open/Closed principle is one of the five principles basic of object-oriented design(OOD) which are defined as the S.O.L.I.D.. The principles of SOLID are guidelines that can be applied while working on software to remove code smells by causing the programmer to refactor the software's source code until it is both legible and extensible. It is typically used with test-driven development, and is part of an overall strategy of agile and adaptive programming<ref name = openclosed_video />.

Motivation

All software systems are subject to change. Thus designing a system which is stable is a very crucial task. When a single change to a program results in a cascade of changes to dependent modules, that program exhibits the undesirable attributes that we have come to associate with “bad” design. The program becomes fragile, rigid, unpredictable and unreusable. The open-closed principle attacks this in a very straightforward way. Open-closed principle states that the code should be designed in such a way that it should not change. Whenever requirements change, the existing code should be extended by adding new code and leave the old working code intact.

Most of the times it easier to write all new code rather thank making changes to existing code. Modifying old code adds the risk of breaking existing functionality. With new code you generally only have to test the new functionality. When you modify old code you have to both test your changes and then perform a set of regression tests to make sure you did not break any of the existing code.

The Open Close Principle states that the design and writing of the code should be done in a way that new functionality should be added with minimum changes in the existing code. The design should be done in a way to allow the adding of new functionality as new classes, keeping as much as possible existing code unchanged. Thus following open-closed principle leads to good software design.

Conclusion

Making a flexible design involves additional time and effort. Open/Closed principle introduces a new level of abstraction increasing the complexity of the code. Hence, principle should be applied in those parts of the code which are most likely to be changed. This principle is at the heart of object oriented design in many ways. This principle motivated many heuristics associated with object oriented design. For example, “all member variables should be private”, or “global variables should be avoided”.

Conformance to this principle yields some of the greatest benefits claimed for object oriented technology; i.e. re-usability and maintainability. Yet conformance to this principle is not achieved simply by using an object oriented programming language. Rather, it requires a dedication on the part of the designer to apply abstraction to those parts of the program that the designer feels are going to be subject to change.

See Also

• Single-Responsibility Principle
• Chain of Responsibility Pattern
• SOLID - Object Oriented Design

References

<references> <ref name = meyer> Meyer, Bertrand (1988). Object-Oriented Software Construction. Prentice Hall. ISBN 0-13-629049-3. </ref> <ref name = openclosed_wiki> http://en.wikipedia.org/wiki/Open/closed_principle Open/closed principle </ref> <ref name = pdf> http://www.objectmentor.com/resources/articles/ocp.pdf The Open-Closed Principle, C++ Report, January 1996 </ref> <ref name = microsoft> http://msdn.microsoft.com/en-us/magazine/cc546578.aspx Patterns in Practice: The Open Closed Principle </ref> <ref name = openclosed_video> http://ruby-toolbox.com/categories/testing_frameworks.html </ref> </references>