21. Design a Car Rental System : Low Level Design Interview Round

Design a Car Rental Service

Design a car rental service . System should support full-day bookings and calculate trip-cost at the end.

Rentals are full-day only and for at least 1 day.
Date format: yyyy-mm-dd (e.g., 2025-08-28).

Core Concepts

A Car has: licensePlate (unique), costPerDay, freeKmsPerDay (free kilometers per day) and costPerKm for extra kilometers that you travel, beyond freeKmsPerDay .
A Booking blocks a car for an inclusive date range [fromDate..tillDate].
A Trip starts with an odometer reading and ends with a final reading and an endDate.
The chargeable day count is computed inclusively:
days = 1 + (effectiveEndDate - fromDate) in whole calendar days.

Examples:
- 2025-08-28 → 2025-08-28 ⇒ 1 day
- 2025-08-28 → 2025-08-30 ⇒ 3 days
Effective end date rule at return:
effectiveEndDate = max(tillDate (from booking), endDate (provided at endTrip))

You always charge up to the later of the two dates.
If any trip ends prematurely then we do open that car for booking trips immediately.
e.g. if trip was booked from 2025-08-06 till 2025-08-12 but it ended on 2025-08-09
then after the trip has ended calls to bookCar() will succeed for that vehicle from 2025-08-10 onwards
To make calculations simple,
Both start and end date of trip will always be in same month and same year

Methods

void addCar(String licensePlate, int costPerDay, int freeKmsPerDay, int costPerKm)

licensePlate is non-blank and unique.
Costs and kilometer values are non-negative integers.

boolean bookCar(String orderId, String carLicensePlate, String fromDate, String tillDate)

orderId is non-blank and unique.
carLicensePlate is non-blank and must refer to an existing car.
Dates use yyyy-mm-dd, fromDate ≤ tillDate.
Booking succeeds only if the car is not already booked for any overlapping date(s) with an existing booking.
- Overlap is inclusive over calendar days, i.e., two ranges [A..B] and [C..D] overlap if A ≤ D and C ≤ B.
Both fromDate and tillDate will always be in same month and same year
Return: true if successfully booked and all rules pass; otherwise false.

void startTrip(String orderId, int odometerReading)

orderId refers to a previously booked order.
odometerReading is the car’s current odometer at handover (positive integer).
Stores the start odometer for later cost calculation.

int endTrip(String orderId, int finalOdometerReading, String endDate)

orderId, finalOdometerReading and endDate are valid.
Compute cost with the Effective End Date Rule and Distance Rule defined above.
Both fromDate from bookCar() and endDate will always be in same month and same year
bookCar() can't be called for endDate, even if it is a delayed trip end.
e.g. if trip ends on 2025-11-09 then bookCar() for that car can only be called from 2025-11-10
Return: the total trip cost as an integer.

Examples

Example A - Within booked window, no extra kilometers

addCar("KA01AB1234", 1200, 100, 10)

bookCar("ORD-1", "KA01AB1234", "2025-08-28", "2025-08-30")  // 3 days (28,29,30)
→ true

startTrip("ORD-1", 5000)

endTrip("ORD-1", 5250, "2025-08-29")  // endDate earlier than tillDate → charge till 2025-08-30
days = 3
tripKms = 5250 - 5000 = 250
freeAllowance = 3 × 100 = 300
extraKms = max(0, 250 - 300) = 0
totalCost = (3 × 1200) + (0 × 10) = 3600
→ returns 3600

Example B - Extended beyond booked window with extra kilometers

addCar("DL09CD4321", 1500, 120, 8)

bookCar("ORD-2", "DL09CD4321", "2025-09-01", "2025-09-02")  // 2 days (1,2)
→ true

startTrip("ORD-2", 20000)

endTrip("ORD-2", 20550, "2025-09-04")  // extended; effectiveEndDate = 2025-09-04
days = 1 + (2025-09-04 - 2025-09-01) = 4
tripKms = 20550 - 20000 = 550
freeAllowance = 4 × 120 = 480
extraKms = max(0, 550 - 480) = 70
totalCost = (4 × 1500) + (70 × 8) = 6000 + 560 = 6560
→ returns 6560

Example C - Overlapping booking rejected

addCar("MH12EF9999", 1000, 80, 12)

bookCar("ORD-3", "MH12EF9999", "2025-08-10", "2025-08-12")  // 10,11,12
→ true

bookCar("ORD-4", "MH12EF9999", "2025-08-12", "2025-08-15")  // overlaps on 12 (inclusive)
→ false

Constraints & Notes

All data is in memory; no persistence or external systems.
Single process; no concurrency control required.
Inputs will not be null. Assume date strings are well-formed yyyy-mm-dd.
Behavior for impossible values (e.g., finalOdometerReading < startOdometerReading) need not be handled, as inputs are stated valid.
Currency and units are abstract; treat all amounts as integers.