Problem definition

A parking lot is a designated area for parking vehicles, commonly found in venues like shopping malls, sports stadiums, and office buildings. It consists of a fixed number of parking spots allocated for different types of vehicles. Each spot is charged based on the duration a vehicle remains parked. Parking time is tracked using a parking ticket issued at the entrance. Upon exit, the customer can pay using either an automated exit panel or through a parking agent, with a credit/debit card or cash as accepted payment methods.

In this LLD interview case study, your focus will be on:

Managing parking for various vehicle types (car, van, truck, motorcycle, etc.) across potentially multiple floors and zones.
Allocating and tracking parking spot availability by type (e.g., compact, large, handicapped, motorcycle).
Issuing and managing parking tickets for tracking entry, parking duration, and exit.
Handling flexible payments at automated exit panels or via human agents, supporting multiple payment methods (cash, card, coupon).
Implementing and enforcing pricing models that may vary by hour, spot type, or vehicle type.
Ensuring real-time tracking of spot occupancy and enabling efficient entry/exit flows, especially during busy periods.
Accommodating special considerations like handicapped spots and overflow situations.

This parking lot system model can be adapted for various venues (malls, stadiums, offices, airports) and is extensible to different business rules (such as prebooking, dynamic pricing, or loyalty programs).

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Expectations from the interviewee

In a typical parking lot system, there are several components each with specific constraints and requirements. The following section provides an overview of some major expectations the interviewer will want an interviewee to discuss in more detail during the interview.

Payment flexibility

One of the most significant attributes of the parking lot system is the payment structure that it provides to its customers. An interviewer would expect you to ask questions like these:

How are customers able to pay at different exit points (i.e., either at the automated exit panel or to the parking agent) and by different methods (cash, credit, coupon)?
If there are multiple floors in the parking lot, how will the system keep track of the customer having already paid on a particular floor rather than at the exit?

Parking spot type

Another topic of discussion that an interviewer would expect you to be aware of is the different parking spot types—handicapped, compact, large, and motorcycle—regarding which you can ask the following questions:

How will the parking capacity of each lot be considered?
What happens when a lot becomes full?
How can one keep track of the free parking spots on each floor if there are multiple floors in the parking lot?
How will the division of the parking spots be carried out among the four different parking spot types in the lot?

Vehicle types

Similar to the parking spot, an interviewer would also expect you to discuss the different vehicle types—car, truck, van, motorcycle—which can have the following set of questions:

How will capacity be allocated for different vehicle types?
If the parking spot of any vehicle type is booked, can a vehicle of another type park in the designated parking spot?

Pricing

We touched upon the payment structure offered by the parking lot system. Now, the pricing model needs to be clarified from the interviewer, and therefore you may ask questions like these:

How will pricing be handled? Should we accommodate having different rates for each hour? For example, customers will have to pay $\$4$ for the first hour, $\$3.5$ for the second and third hours, and $\$2.5$ for all the subsequent hours.
Will the pricing be the same for the different vehicle types?

Design approach

We will design this parking lot system using a bottom-up design approach. For this purpose, we will follow the steps below:

First, we’ll identify the core entities such as Vehicle, ParkingSpot, ParkingTicket, and Payment, and define their primary responsibilities.
Next, we’ll model how vehicles are assigned to appropriate parking spots based on type and availability, how entry/exit is managed, and how parking duration is tracked.
We’ll design the payment system to support different payment methods and flexible pricing rules, ensuring secure and accurate fee calculation.
We’ll ensure the system supports scalability (multiple floors, high traffic), real-time updates on availability, and follows SOLID principles for maintainability.

Later in this case study, diagrams and code will be used to illustrate the main workflows and class structures.

Design pattern

During an interview, it is always a good practice to discuss the design patterns that a parking lot system falls under. Stating the design patterns gives the interviewer a positive impression and shows that the interviewee is well-versed in the advanced concepts of object-oriented design.

Try to answer the following question. If you are not familiar with design patterns, don’t worry! You can learn about them by asking questions like, “Define design patterns.”