Design object model for an elevator system
Company: Amazon
Role: Software Engineer
Category: Software Engineering Fundamentals
Difficulty: medium
Interview Round: Onsite
Design the object-oriented model for an elevator (lift) system in a multi-floor building that may have **multiple elevators**.
The system should support:
- **External requests**: people on each floor can press Up/Down call buttons.
- **Internal requests**: passengers inside the elevator can select a destination floor.
- An elevator can move up/down, open/close doors, and stop at requested floors.
- A **scheduler/controller** decides which elevator should handle which requests (especially when there are multiple elevators).
**Tasks**
1. Identify and describe the **main classes**, their responsibilities, and relationships. Examples might include: `Elevator`, `ElevatorController`, `Floor`, `Request`, `Button`, etc.
2. Explain how the system handles **request flow** end-to-end:
- A user presses an Up/Down button on a floor.
- An elevator is assigned and moves to that floor.
- The user selects a destination floor inside the elevator.
- The elevator services multiple requests while moving.
3. Propose a **scheduling strategy** (algorithm) for choosing which elevator should handle new requests (at a high level, not full code). Discuss how it works in typical scenarios (e.g., many people going up in rush hour).
4. Discuss how your design can handle **edge cases and extensions**, such as:
- Elevators going into maintenance mode.
- Overload (too many people inside).
- Emergency stop or fire mode.
- Adding more elevators or floors in the future.
You may use UML-style notation or plain text descriptions for classes and interactions.
Quick Answer: This question evaluates object-oriented design and system-architecture skills for modeling a multi-elevator control system, including class responsibilities, interactions, request flow, and scheduling policies.
