Design an Elevator System

Requirements

• A building has multiple elevators and multiple floors.
• A user on a floor requests a direction (up/down); a user inside an elevator requests a destination floor.
• The system must decide which elevator responds to a given request (scheduling), and each elevator must process its queued stops in a sensible order.
• Each floor's display should reflect the assigned elevator's position/direction as it changes.

Key classes to identify before you look at the reference

This is the most commonly cited example for combining multiple patterns in one design. Consider separately: (1) what decides which elevator gets a request — and how would you swap that logic for a different scheduling algorithm without touching the rest of the system? (2) what notifies floor displays when an elevator's state changes, without the elevator needing to know which displays exist?