state

State

Intent

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

Also Known As

Objects for States

Applicability

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

Participants

• Context (TCPConnection)
  • defines the interface of interest to clients
  • maintains an instance of a ConcreteState subclass that defines the current state
• State (TCPState)
  • defines an interface for encapsulating behavior associated with a particular state of the Context
• ConcreteState subclasses (TCPEstablished, TCPListen, TCPClosed)
  • each subclass implements a behavior associated with a state of the Context

Related Patterns

The Flyweight pattern explains when and how State objects can be shared.
State objects are often Singletons.