StatePattern

StatePattern, commonly known as the State pattern, is a behavioral design pattern that allows an object to alter its behavior when its internal state changes. From the outside, the object may appear to change its class, while in fact it delegates requests to different state objects. The pattern encapsulates state-specific behavior within separate ConcreteState classes and manages transitions between states to avoid large conditional logic.

Structure and operation: A Context object keeps a reference to a State object that defines the interface for handling requests. ConcreteState implementations provide the behavior for each state. The Context delegates incoming requests to the current State, and the State may request the Context to transition to another state by calling setState.

When to use: Use the StatePattern when an object’s behavior depends on its state, and the set of states and transitions is well-defined. It is especially helpful if the state contains significant behavior that would otherwise be expressed with lengthy conditionals, or when states should be swappable at runtime without modifying the Context.

Benefits and trade-offs: Benefits include better organization of state-specific logic, easier maintenance, and easier extension by adding new states. Trade-offs include an increase in the number of classes and the need to manage transitions carefully; for simple state machines the pattern may be overkill.

Relation and history: The StatePattern is one of the classic design patterns described by Gamma, Helm, Johnson, and Vlissides in Design Patterns (1994). It is related to the Strategy pattern and to finite-state machines, since both use interchangeable behaviors depending on state. Common examples include protocol connections, user interface modes, and workflow engines.