Class hierarchies should be deep and narrow

A good class hierarchy design should be several layers deep. In a good design, a base class should not have large number of direct children classes (i.e. a superclass has 28 subclasses). If a class hierarchy is not deep but very wide, it is a sign that a change is needed.

Solution for solving this problem is to find some common behaviors from the children sibling classes, and then extract these common behaviors into a parent class under the original base parent class.

Riel's heuristic Favour deep hierarchies is derived from this idea.

See also

• Favour deep hierarchies
• Design maxims