lld-parking-lot

A thread-safe parking lot in plain Java, built the way a low-level-design interview expects: requirements, entities, patterns, concurrency, extensions. Companion to Low-Level Design Interview in Java: Parking Lot System, Step by Step.

No framework, no runtime dependencies. JUnit 6 is used for tests only.

Versions

Component	Version	Verified against
JDK	25.0.4.1+1 (Temurin LTS); also compiled and run with `--release 21` on OpenJDK 21	`java -version`; the walkthrough output is byte-identical on both
JUnit Jupiter	6.1.3	`org.junit.jupiter:junit-jupiter` metadata on Maven Central
Maven	3.9.11	`mvn -version`

Run it

cd lld-parking-lot
./scripts/run.sh          # compiles with javac, prints the walkthrough (no Maven needed)
mvn test                  # 10 tests, including two concurrency tests
./scripts/run-all.sh      # regenerates everything in output/

Layout

src/main/java/com/ankurm/interviewlab/parking/
  VehicleType, SpotSize           enums; SpotSize.canHold(VehicleType) is the only "does it fit" rule
  Vehicle, Ticket, Receipt        immutable records
  ParkingSpot                     occupancy is one AtomicReference compare-and-set
  ParkingFloor                    ordered spots
  AllocationStrategy              Strategy: NearestFirstStrategy, BestFitStrategy
  PricingStrategy                 Strategy: HourlyPricing (grace period, partial hours round up)
  AvailabilityListener            Observer: the entrance display board
  ParkingException                LotFull, AlreadyParked, UnknownTicket
  ParkingLot                      the aggregate: park(), exit(), freeSpots()
  Demo                            prints output/01-walkthrough.txt
src/test/java/.../ParkingLotTest  10 tests; Clock is injected so nothing sleeps
output/                           captured real output

Captured output

File	What it shows
`output/01-walkthrough.txt`	Enter, leave, fail three ways, and the two allocation strategies placing the same motorcycle differently
`output/02-test-results.txt`	One line per test, generated from the Surefire XML report

License

MIT, see the repository root.

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