Object-Oriented Design of an Elevator Control System
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An object-oriented design question asking the candidate to model an elevator control system: its classes, their responsibilities, and how a button press becomes an elevator stop. It tests requirement scoping, a clear per-car state model, stop ordering and dispatch policies that can be swapped out, handling of out-of-service cars and duplicate requests, and testability.
Design an elevator control system using object-oriented design. Identify the classes and their responsibilities, show how a button press becomes an elevator stopping at a floor, and implement the core of the control logic in TypeScript or another language of your choice.
The round was reported only as an object-oriented elevator design problem. The setup below is a standard starting point to confirm with the interviewer, not reported requirements:
- A building has floors numbered from a lowest to a highest floor, and one or more elevator cars.
- Each floor has hall buttons for up and down. The top floor has only down, and the bottom floor has only up.
- Each car has a panel of floor buttons.
- A controller decides which car answers each hall call, and each car moves one floor at a time, stops, and opens its doors.
Constraints and Clarifications
- Treat the physical elevator as a simulation that advances in steps, so the logic can be tested without real hardware or real time.
- Scheduling goals, special operating modes, and capacity rules are open until clarified. Choose defaults and state them.
Clarifying Questions
- How many floors and cars should the design support, and must it work for a single car as well as a group of cars?
- What should the scheduling optimize: passenger waiting time, travel time, or energy?
- Are there capacity or weight limits, and what happens when a car is full?
- Which special modes are needed, such as maintenance, emergency stop, or fire service?
- What should happen to requests assigned to a car that goes out of service?
Hints
Hint 1
Separate choosing a car from ordering stops: Consider which object decides which car answers a hall call, and which object decides the order in which one car visits its stops. Keeping them separate makes each policy replaceable.
Follow-up questions
- How would you change car assignment for heavy morning traffic from the lobby going up?
- How would you reassign a failed car's hall calls without losing any of them?
- How would you add a rule that doors stay open while an obstruction sensor is triggered?
- How would you test that no request waits forever?
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