Introduction to OOP
Chapter 4: Classes and Methods:
Class Data Fields
Idea is that all instances of a class can share a common data field. Simple idea, but how to resolve the following paradox. All instances have the same behavior:
Either they all initialize the common area, which seems bad, or
Nobody initializes the common area, which is also bad.
Different languages use a variety of mechanisms to get around this. See text for details.
, Slide 32