Editing State pattern (section)

==Overview==
[[File:w3sDesign State Design Pattern UML.jpg|thumb|right|A sample UML class and sequence diagram for the State design pattern.<ref>{{cite web |title=The State design pattern – Structure and Collaboration |url=http://w3sdesign.com/?gr=b08&ugr=struct |website=w3sDesign.com |access-date=2017-08-12}}</ref>]]
The state design pattern is one of twenty-three [[Design Patterns|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.<ref name="GoF">{{cite book |author=Erich Gamma |author2=Richard Helm |author3=Ralph Johnson |author4=John Vlissides |title=Design Patterns: Elements of Reusable Object-Oriented Software |year=1994 |publisher=Addison Wesley |isbn=0-201-63361-2 |pages=[https://archive.org/details/designpatternsel00gamm/page/305 305ff] |url-access=registration |url=https://archive.org/details/designpatternsel00gamm/page/305 }}</ref>

The state pattern is set to solve two main problems:<ref>{{cite web |title=The State design pattern - Problem, Solution, and Applicability |url=http://w3sdesign.com/?gr=b08&ugr=proble |website=w3sDesign.com |access-date=2017-08-12}}</ref>
* 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.