SOLID — Interface Segregation Principle: Clients Should Not Depend on Unused Methods

A fat interface forces implementors to provide methods they do not use. A class implementing IWorker with both work() and eat() must implement eat() even if it is a Robot that doesn't eat.

Split large interfaces into smaller, focused ones. Clients depend only on the interfaces they use.

// Bad: fat interface
interface IWorker {
  work(): void;
  eat(): void;
  sleep(): void;
}
// Robot must implement eat() and sleep() with stubs — violation

// Good: segregated interfaces
interface IWorkable { work(): void; }
interface IFeedable { eat(): void; }
interface ISleepable { sleep(): void; }

class Human implements IWorkable, IFeedable, ISleepable {
  work() { console.log('working'); }
  eat()  { console.log('eating'); }
  sleep() { console.log('sleeping'); }
}

class Robot implements IWorkable {
  work() { console.log('robot working'); }
  // no eat() or sleep() required
}

// A function that only needs work() depends only on IWorkable
function manage(worker: IWorkable) { worker.work(); }
manage(new Robot()); // no problem

ISP reduces coupling: a change to IFeedable does not require recompiling or retesting robots. It also prevents forcing stub implementations that violate LSP.