I was lucky enough to get an equivalent piece of advice from "Object Oriented Programming" by Peter Coad & Jill Nicola ( http://amzn.com/013032616X http://amzn.

I was lucky enough to get an equivalent piece of advice from "Object Oriented Programming" by Peter Coad & Jill Nicola ( http://amzn.com/013032616X http://amzn.com/013032616X ) which was the main textbook for an OO course during my CS studies. There are good examples there that illustrate this and many other useful rules (of thumb) of OO modeling.