Chapter 6. Object-oriented principles

Once there was a large, heavy, complex web application with lots of modules, bells and whistles, or even timpani and trumpets. It was reasonably successful, but needed to be adapted to a new customer’s needs. The customer needed something with fewer features but with a specific look and feel. The look and feel was well defined: There was an HTML file containing all the styling and layout that was needed.

The existing application had flexibility built in so that a web designer could change the layout templates to create a completely new layout. Everything was based on CSS and XSLT, so all it should take, in theory at least, was to copy all the style sheets and modify them. Unfortunately, that was not what was needed for this particular new customer. The task required tweaking existing features, removing some, and squeezing it into the layout that had been specified. Partly because of the size of the application, and the fact that the new required layout was simpler, it was easier to discard the old templates and use the HTML file as a starting point for new templates. So as far as the new requirements were concerned, the work that had been put into making it flexible was mostly wasted.

6.1. Principles and patterns

6.2. The open-closed principle (OCP)

6.3. The single-responsibility principle (SRP)

6.4. The dependency-inversion principle (DIP)

6.5. Layered designs

6.6. Summary