widgetness

A facetious example to illustrate the point: I am required to implement DogWidgets and CatWidgets and I have two choices - separate the Widgetness into some kind of base class, or copy-paste.

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.

However, now every time a change is made to CatWidget, if the design is good then it should be fairly obvious whether it's a change to Widgetness, or a change to Catness.

> (...) so we eventually end up with two divergent implementations of Widgetness.

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.

However, now every time a change is made to CatWidget, if the design is good then it should be fairly obvious whether it's a change to Widgetness, or a change to Catness.

> (...) so we eventually end up with two divergent implementations of Widgetness.

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.