Design Patterns Java™ Workbook

Chapter 22. State

Figure 22.1. A carousel door provides one-touch control with a single button that changes

the door's state.

The diagram in Figure 22.1 is a UML state machine. Such diagrams can be much more
informative than a corresponding textual description.

CHALLENGE 22.1

Suppose that you open the door and place a material bin in the doorway. Is there

a way to make the door begin closing without waiting for it to time out?

You can supply a Door_1 object that the carousel software will update with state changes in
the carousel. (The underscore in the class name is a hint that we will soon refactor this class.)
Figure 22.2 shows the Door_1 class.