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

• An object should change its behavior when its internal state changes.
• State-specific behavior should be defined independently.
  • New states should be added
  • the behavior of existing states should be changed independently

• Define separate State objects that encapsulate state-specific behavior for each state.
• A class delegates state-specific behavior to its current state object instead of implementing state-specific behavior directly.

• Context
  • maintains an instance of ConcreteState that defines the current state.
• State
  • defines an interface for encapsulating the behavior associated with a particular state of the Context.
• ConcreteState
  • each subclass implements a behavior associated with a state of the Context.

• Context delegates state-specific requests to the current ConcreteState object.
• A context may pass itself as an argument to the State object handling the request. This lets the State object access the context if necessary.
• Context is the primary interface for clients. Clients can configure a context with State objects. Once a context is configured, its clients don't have to deal with the State objects directly.
• Either Context or the ConcreteState subclasses can decide which state succeeds another and under what circumstances.

• Organizes the code related to particular states into separate classes.
• It makes state transitions explicit by using separate objects for different states.
• Simplifies the code of the context
• State objects can be shared, if they have no instance variables (cf. Flyweight with no intrinsic state but only behavior)

• Can be overkill if a state machine has only a few states or rarely changes.

Definition

Usage

• Flyweight pattern explains when and how State objects can be shared.