WardCunningham the founder
DRY Dont Repeat Yourself
Once And Only Once
If It Is Working Dont Change
An Introduction To Database Systems
A class has a single responsibility: it does it all, does it well, and does it only.
One of the criteria I use is to try to describe a class in 25 words or less, and not to use "and" or "or". If I can't do this, then I may actually have more than one class.