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Object-Oriented Parking Lot Design with Check-In and Check-Out

UberSoftware Engineer · Onsite
HardSoftware EngineerOnsiteSoftware Engineering Fundamentals

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An object-oriented design interview question asking you to model a parking lot and implement working check-in and check-out methods. It tests class design and relationships, fast free-spot lookup across vehicle sizes, ticket handling, fees, edge cases, and thread safety.

Design a parking lot in an object-oriented style and implement its two core operations:

  • check_in: a vehicle arrives. Assign it a free spot it fits in, and give the driver something to present when leaving, such as a ticket. If no suitable spot is free, report that clearly.
  • check_out: the vehicle leaves. Release its spot and close out the visit.

Write runnable code for these methods and the classes they need, and handle the edge cases.

Clarifying Questions

  • Which vehicle types and spot sizes exist, and can a smaller vehicle use a larger spot?
  • Does check-out compute a fee? If so, what is the pricing rule?
  • What does check-in return: a ticket, a spot, or an error when the lot is full?
  • Does the lot have several levels, and does the spot assignment policy matter (nearest to the entrance, lowest level, smallest fitting spot)?
  • Can several entrances and exits call these methods at the same time?
  • What should happen if a vehicle that is already parked checks in again, or if an unknown or already used ticket is presented at check-out?

Hints

Hint 1

Start from the two methods: Write the signatures of check-in and check-out first, and list what each one must look up. That tells you which classes and which indexes you actually need.

Hint 2

Finding a free spot fast: Scanning every spot on each check-in works but is slow for a large lot. Consider what you could maintain so that "a free spot that fits this vehicle" is cheap to find and cheap to give back.

Follow-up questions

  • Add pricing that differs by vehicle type and time of day. Which class changes?
  • Two entrances check in vehicles at the same moment. How do you guarantee they never get the same spot?
  • Add a display that shows free spots per level and per size.
  • How would you support reservations made ahead of arrival?