dogwidget

If I chose instead to implement Widget, and derive DogWidget and CatWidget, then yes, I am imposing some additional burden on future developers to learn how the code is structured.

Rolling CatWidget's and DogWidget's Widgetness back together into a base class is now a very difficult refactoring job that is determined to take twice as long as just copy-pasting DogWidget and making Bird changes; doing the right thing by the code doesn't make economic sense.

The bug turns out to be partly due to the original Widget implementation, and partly due to Joe's changes - now Jolene not only has to fix CatWidget, but also examine DogWidget carefully to determine if a) the bug affects DogWidget too, and how, and b) whether or not to fix it, and how (which may be different from the CatWidget fix).

Bug fixes and improvements to Widget now automatically filter down to both (and all future) Widgets; if Widgets need a colour, then even if we only want to give it to CatWidgets at the moment, we can stub it out or provide a default value for DogWidget, and that thought process also forces us to examine our design instead of ignoring DogWidget altogether, which helps to keep the design current and relevant.

Now Joe Junior Developer comes along and over a period of several months, is asked to make a series of changes only to CatWidgets; some of these pertain to Widgetness, but Joe is not experienced or familiar enough with the code to understand that, and anyway has not been told to change DogWidget, so we eventually end up with two divergent implementations of Widgetness.