Design Patterns Java™ Workbook

Modeling States

When you model an object whose state is important, you may find that you have a variable that tracks how the object should behave, depending on its state. This variable may appear in complex, cascading if statements that focus on how to react to the events that an object can experience. One problem with this approach to modeling state is that if statements can become complex. Another problem is that when you adjust how you model the state, you often have to adjust if statements in several methods. The STATE pattern offers a cleaner, simpler approach, using a distributed operation.

Consider the Oozinoz software that models the state of a carousel door. A carousel is a large, smart rack that accepts material through a doorway and ...