Chapter 6. Bridgeinput queue overflows and the machines signals an alarm. Figure 6.5 shows a design that
provides for testing classes.
Figure 6.5. In this factoring, subclasses of MachineController exist for various types
of machines and for various types of controllers.Introducing a TestController class turns out to force you to create three new classes, as
test controllers for star presses and shell assemblers implement getQueueMax() differently.
This is a problem because you'd like to be able to add new classes individually. The problem
arises because the hierarchy is factored according to two principles: Each machine needs its
own controller, and different types of controllers are used for normal operation and for
testing. You really need a class for every combination of machine type and controller type—a
maintenance nightmare. You can solve the problem by separating the MachineController
abstraction from its implementation.
To refactor a hierarchy with an abstract class at its top into a bridge, do the following.
- Move the abstract operations in the superclass into an interface.
- Define implementation classes that provide different implementations of the interface.
- Redefine the remaining operations in the abstract class as operations on an instance of
the new interface.