The blog post this week discusses my favorite principle in software development, the open and closed principle. The open and closed principle encourages independencies in software development. It states the following as outlined in the article: Class behavior should be open for extension but closed for modifications. This can be separated into two parts. As explained in the article, the first part is about the behavior of a class. Extensions of behavior allows for changes in behavior to be added, deleted or modified without affecting the other behaviors. This is what is meant to be independent. Thus, the 1st part of the statement signifies changes in behavior which signifies changes in source code. The second part of the statement states that class behavior should be closed for modifications. Is this a contradiction? Modifications means changes to source code, which the second statement encourages developers not to change. This means that class behavior should be independent and not need to be modified when other parts of its behavior are changed. So, when extending a class behaviors, the source codes for other behaviors should not be changed. The other definition introduced in the article is: Software entities(classes, modules, functions, etc.) should be open for extension, but closed for modifications. The explanation for the principle is as I have described previous just above that it encourages independence in software methods.
The article, however, expresses confusion for both definitions because the two sides of the principle is a contradiction, however I think it did a pretty good job in its explanations and examples. As explained in the article, the main idea behind the OCP is that the behavior of the system can change by replacing “entities that constitute it with alternative implementations,” but that the other behaviors can be independent and therefore need not be changed. The example given is that for calculating taxes. As stated, take for example the TaxCalculator interface for calculating taxes. In this system, if we replace the UsTaxCalculator with the UkTaxCalculator, this does not require modifications of existing logic, but that it provides for new implementations to an existing interface. Overall, the main idea is that you can add behaviors to systems be adding new code, but the existing ones do not need to be changed. This creates separability so that system behavior can be easily modified and changed.
Finally, I will close this post with a discussion about 3 rules of thumb suggested in the article that can help a developer to determine when to use the open closed principle. The first is to add extension points to classes that implement requirements that are intrinsically unstable due to instability of the system. This helps to clean the code which makes it easier for modifications of system behavior. The second is to not add more preliminary extension points. Finally, the third is to refactor parts of the code and to introduce additional extension points when the parts are unstable.
Overall I chose this article in order to compare my understanding of the OCP with another software developer. In my opinion, the OCP is based on indenpendence of methods, functions and classes. Although the article indirectly refers to this point, still I think it misses to introduce this point directly.