Managing Coupling Between Superclasses and Subclasses

Bicycle now contains most of the abstract bicycle behavior. It has code to manage overall bike size, chain, and tire size, and its structure invites subclasses to supply common defaults for these attributes. The superclass is almost complete; it’s missing only an implementation of spares.

This spares superclass implementation can be written in a number of ways; the alternatives vary in how tightly they couple the subclasses and superclasses together. Managing coupling is important; tightly coupled classes stick together and may be impossible to change independently.