One key abstraction
- A class should capture one and only one key abstraction. --Riel's Heuristic 2.8, Arthur Riel 1996
A class should represent one key abstraction of the domain model, no more and no less. A key abstraction is generally a noun in a requirements statement. If a key abstraction maps to more than one class, the designer may be representing each function of the abstraction as a class. If several key abstractions map to one class, the designer may be creating a centralized system and needs to split the class into several smaller classes.