Freek Lijten has posted the second part of his series looking at the SOLID development methodology. In this latest post his looks at the second letter in the acronym - "O" for "Open Closed Principle."
Software that requires an enormous amount of changes to implement one new feature or fix a bug is unstable and should be considered as "bad". Software should be designed so, that in case of a new feature, no existing classes should have to change. In other words: it is closed for modification. Existing software may be extended to achieve new features however.
He starts off with a "What" section explaining a bit more about what this open/closed means for your code and gets into an example showing it in a more practical way. He shows how to take the principle and refactor an API connector class to pass in the object it needs (Bike) and use that to get information (rather than just passing in the data). He uses a Factory to get the object type he needs based on the Bike type.