Abstract Class- An Object Oriented View

We usually think of classes as being complete definitions. However, there are situations where incomplete definitions are useful, and classes that represent these incomplete definitions are equally useful. For example, in everyday conversation, we might talk about such items as bank accounts, insurance policies, and houses. In object-oriented thinking, we often isolate useful, but incomplete, concepts such as these into their own special classes.

Abstract classes are classes that embody coherent and cohesive, but incomplete, concepts, and in turn, make these characteristics available to their specializations via inheritance. People sometimes use the terms "partial type" and "abstract superclass" as synonyms for abstract class. While we would never create instances of abstract classes, we most certainly would make their individual characteristics available to more specialized classes via inheritance.

For example, consider the concept of an automobile. On one hand, most people know what an automobile is. On the other hand, "automobile" is not a complete definition for any vehicle. It would be quite accurate to describe "automobile" as the set of characteristics that make a thing an automobile, in other words, the "essence of automobile-ness."