State Pattern

Allow an object to alter its behavior when its internal states changes. The object will appear to change its class.

defines the interface of interest to clients.
maintains an instance of a ConcreteState subclass that defines the current state.
this participant can not be read-only.
this participant can not be an interface.

defines an interface for encapsulating the behavior associated with a particular state of the Context.
this participant can not be read-only.

each Concrete State implements a behavior associated with a state of the Context.
this participant can not be read-only.

here you can specify the name of an attribute where State Context keeps a reference to the current Concrete State object.

when selected, will create special JavaDoc links in the code between pattern participants, describing their relationships.

Use the State pattern in either of the following cases:

an object's behavior depends on its state, and it must change its behavior at run-time depending on that state.
operations have large, multipart conditional statements that depend on the object's state. This state is usually represented by one or more enumerated constants. Often, several operations will contain this same conditional structure. The State pattern puts each branch of the conditional in a separate class. This lets you treat the object's state as an object in its own right that can vary independently from other objects.