> For AI agents: the complete documentation index is available at https://nhanbe.id.vn/llms.txt, the full documentation bundle is available at https://nhanbe.id.vn/llms-full.txt.

# State

## Overview

Allow an object to alter its behavior when its internal state changes.

## When to use

- Behavior varies based on state.
- You want to avoid large conditionals.

## Java example

```java
interface State {
    String render();
}

class DraftState implements State {
    public String render() { return "draft"; }
}

class Document {
    private State state = new DraftState();
    void setState(State state) { this.state = state; }
    String render() { return state.render(); }
}
```

## TypeScript example

```ts
interface State {
  render(): string;
}

class DraftState implements State {
  render(): string { return "draft"; }
}

class Document {
  private state: State = new DraftState();
  setState(state: State): void { this.state = state; }
  render(): string { return this.state.render(); }
}
```

## Pros and cons

Pros:

- Removes complex conditional logic.
- State transitions are explicit.

Cons:

- More classes for each state.
- Can be overkill for simple cases.

## Common pitfalls

- States that leak knowledge of others.
- Hidden transitions spread across code.

## Related patterns

- [Strategy](https://nhanbe.id.vn/design-patterns/behavioral/strategy.md)
- [Memento](https://nhanbe.id.vn/design-patterns/behavioral/memento.md)