State Pattern

# State

- Purpose
- Structure
- Applications
- Consequences

## Purpose

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

## Structure

- Context : defines the interface of interest to clients. maintains an instance of a ConcreteState subclass that defines the current state.
- State : defines an interface for encapsulating the behavior associated with a particular state of the Context.
- ConcreteState Subclass : each subclass implements a behavior associated with a state of the Context.

## Applications

- 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.

## Consequences

- It localizes state-specific behavior and partitions behavior for different states. The State pattern puts all behavior associated with a particular state into one object. Because all state-specific code lives in a State subclass, new states and transitions can be added easily by defining new subclasses.
- It makes state transitions explicit. When an object defines its current state solely in terms of internal data values, its state transitions have no explicit representation; they only show up as
  assignments to some variables. Introducing separate objects for different states makes the transitions more explicit. Also, State objects can protect the Context from inconsistent internal
  states, because state transitions are atomic from the Context's perspective—they happen by rebinding one variable (the Context's State object variable), not several .
- State objects can be shared. If State objects have no instance variables—that is, the state they represent is encoded entirely in their type—then contexts can share a State object. When states are shared in this way, they are essentially flyweights with no intrinsic state, only behavior.