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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.