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State Pattern

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The State pattern is a GoF behavioral pattern that groups behavior by state and lets a component delegate requests to its current state object. A document might allow editing while it is a draft, reject edits during review, and reject further submissions or approvals once published. Each state implementation then decides whether to accept the same Edit() or Approve() request. A login-failure count, a stored variable, and the body being edited are all ~~state, but storing these values alone does not make a design an instance of the State~~ ~~pattern~~. [wikiLink] covers the broader concept. This article explains ~~how~~ to ~~separate~~ behavior ~~that~~ ~~depends~~ on ~~the~~ ~~current~~ ~~state~~.

The State pattern is a GoF behavioral pattern that groups behavior by state and lets a component delegate requests to its current state object. A document might allow editing while it is a draft, reject edits during review, and reject further submissions or approvals once published. Each state implementation then decides whether to accept the same Edit() or Approve() request. Forwarding a request to another object's method in this way is called delegation. A login-failure count, a stored variable, and the body being edited are all program state. [wikiLink] covers the broader concept. This article explains where to group behavior when the same request needs different handling in different states.

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~~Treating~~ ~~these~~ ~~terms~~ as ~~interchangeable~~ ~~would~~ ~~make~~ ~~any~~ ~~order~~ with a ~~status~~ ~~field~~ an ~~example~~ of the State pattern.

A document-review feature needs to record its current stage, decide which requests are allowed there, and define what happens after each request. These are decisions about data and behavior. Whether to handle those rules with conditionals or a transition table, or group them in state-specific implementations, is a separate decision about code structure. We distinguish state values, state machines, and the State pattern to keep these decisions clear.