The state pattern is a

behavioral Behavior (American English) or behaviour (British English) is the range of actions and mannerisms made by individuals, organisms, systems or artificial entities in some environment. These systems can include other systems or organisms as well ...

software design pattern In software engineering, a software design pattern is a general, reusable solution to a commonly occurring problem within a given context in software design. It is not a finished design that can be transformed directly into source or machine code ...

that allows an object to alter its behavior when its internal state changes. This pattern is close to the concept of

finite-state machine A finite-state machine (FSM) or finite-state automaton (FSA, plural: ''automata''), finite automaton, or simply a state machine, is a mathematical model of computation. It is an abstract machine that can be in exactly one of a finite number o ...

s. The state pattern can be interpreted as a

strategy pattern In computer programming, the strategy pattern (also known as the policy pattern) is a behavioral software design pattern that enables selecting an algorithm at runtime. Instead of implementing a single algorithm directly, code receives run-time in ...

, which is able to switch a strategy through invocations of methods defined in the pattern's interface. The state pattern is used in

computer programming Computer programming is the process of performing a particular computation (or more generally, accomplishing a specific computing result), usually by designing and building an executable computer program. Programming involves tasks such as ana ...

to encapsulate varying behavior for the same

object Object may refer to: General meanings * Object (philosophy), a thing, being, or concept ** Object (abstract), an object which does not exist at any particular time or place ** Physical object, an identifiable collection of matter * Goal, an ...

, based on its internal state. This can be a cleaner way for an object to change its behavior at runtime without resorting to conditional statements and thus improve maintainability.

Overview

The state design pattern is one of twenty-three design patterns documented by the Gang of Four that describe how to solve recurring design problems. Such problems cover the design of flexible and reusable object-oriented software, such as objects that are easy to implement, change, test, and reuse. The state pattern is set to solve two main problems: * An object should change its behavior when its internal state changes. * State-specific behavior should be defined independently. That is, adding new states should not affect the behavior of existing states. Implementing state-specific behavior directly within a class is inflexible because it commits the class to a particular behavior and makes it impossible to add a new state or change the behavior of an existing state later, independently from the class, without changing the class. In this, the pattern describes two solutions: * Define separate (state) objects that encapsulate state-specific behavior for each state. That is, define an interface (state) for performing state-specific behavior, and define classes that implement the interface for each state. * A class delegates state-specific behavior to its current state object instead of implementing state-specific behavior directly. This makes a class independent of how state-specific behavior is implemented. New states can be added by defining new state classes. A class can change its behavior at run-time by changing its current state object.

Structure

In the accompanying

Unified Modeling Language The Unified Modeling Language (UML) is a general-purpose, developmental modeling language in the field of software engineering that is intended to provide a standard way to visualize the design of a system. The creation of UML was originally m ...

(UML)

class diagram In software engineering, a class diagram in the Unified Modeling Language (UML) is a type of static structure diagram that describes the structure of a system by showing the system's classes, their attributes, operations (or methods), and the rela ...

, the Context class doesn't implement state-specific behavior directly. Instead, Context refers to the State interface for performing state-specific behavior (state.handle()), which makes Context independent of how state-specific behavior is implemented. The ConcreteStateA and ConcreteStateB classes implement the State interface, that is, implement (encapsulate) the state-specific behavior for each state. The UML

sequence diagram A sequence diagram or system sequence diagram (SSD) shows process interactions arranged in time sequence in the field of software engineering. It depicts the processes involved and the sequence of messages exchanged between the processes needed ...

shows the run-time interactions: The Context object delegates state-specific behavior to different State objects. First, Context calls handle(this) on its current (initial) state object (ConcreteStateA), which performs the operation and calls setState(ConcreteStateB) on Context to change context's current state to ConcreteStateB. The next time, Context again calls handle(this) on its current state object (ConcreteStateB), which performs the operation and changes context's current state to ConcreteStateA.

References

{{Design Patterns Patterns Software design patterns Articles with example Java code