State - Design Patterns Java - page 183

CHALLENGE 22.5
Complete the class diagram in Figure 22.4 to show a design that moves the door
states to an interface.
Figure 22.4. Complete this diagram to shows door states as constants defined by
the DoorConstants interface.
Summary
Generally speaking, the state of an object depends on the collective value of the object's
instance variables. In some cases, an object's state can be a prominent aspect of a class's
behavior. This often occurs when an object is modeling a real-world entity whose state is
important. In such a situation, complex logic that depends on the object's state may appear in
many methods. You can simplify such code by moving state-specific behavior to a hierarchy
of state objects. This lets each state class contain the behavior for one state in the domain. It
also allows the state classes to correspond directly to states in a state machine.
To handle transitions between states, you can let states retain a context reference that contains
a set of states. Alternatively, you can pass around, in state transition calls, the object whose
state is changing. Regardless of how you manage state transitions, the S TATE pattern
simplifies your code by distributing an operation across a collection of classes that represent
an object's various states.