#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

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

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
• Memento