357Chapter 10
CHAPTER 10
State Pattern 10
A State without the means of some change is without
the means of its conservation.
—Edmund Burke
All modern revolutions have ended in a reinforcement
of the power of the State.
—Albert CamusDesign Pattern to Create a State Machine
The State design pattern focuses on the different states in an application, transitions
between states, and the different behaviors within a state. Looking at a simple light
switch application, we can see two states, On and Off. In the Off state, the light is
not illuminated, and in the On state, the light illuminates. Further, the light switch
transitions from the Off state to the On state using a method that changes the appli-
cation’s state—flipping the switch. Likewise, it transitions from On to Off with a dif-
ferent transition and method. An interface holds the transitions, and each state
implements the transitions as methods unique to the state. Each method is imple-
mented differently depending on the context of its use. So, a method,
illuminateLight( ), for example, would do one thing in the Off state and something
entirely different in the On state, even thoughilluminateLight( )method is part of
both states.
Key Features
The following key features characterize the State design pattern:
- States exist internally as part of an object.
- Objects change in certain ways when states change. Objects may appear to
change classes, but they’re changing behavior that is part of the class. - Each state’s behavior depends on the current state of other states in the object.