# 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](https://ankurm.com/low-level-design-interview-parking-lot-system-java/).

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

```bash
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.