State Pattern From GoF

Chris Bushell 7th July 2010

Motivation

“Allow an object to alter its behavior when its internal state changes”

Code

Applicability

• Behavior depends on state• Operations have large conditional statements

which depend on state

Participants

Collaborations

• Context and Concrete State• Context is primary interface for clients

Consequences

• Localizes state specific behavior. Avoids code duplication

• State changes are well defined, atomic operations

• State objects can be shared

Implementation

• Context or Concrete State defines transitions?• Table based approach to defining transitions• Singleton, Flyweight