Question

Design a Parking Lot.

Solution

Class hierarchy:

1. A parking lot has multiple Levels.
2. A Level has multiple Parking Spot.
3. A Spot can park motorcycle, car or bus, which all belongs to Vehicle.

Implement methods:

1. Vehicle.parkInSpot(Spot s)
2. Vehicle.leaveSpot(Spot s)
3. Vehicle.canFitIn(Spot s)
4. ParkingLot.parkVehicle(Vehicle v)
5. Level.parkVehicle(Vehicle v)
6. Level.parkVehicleAtSpot(Vehicle v, Spot s)
7. Level.findAvailableSpot(VehicleType vt)

ParkingLot Class is just a wrapper class of Levels. By doing this, we seperated out parking logic from other broader actions (like Spot management).

The code below is from CC150v5. Its design is a bit strange (a car can occupy multiple spots), so just use this code as a guide but not a reference.

Code

ParkingLot.java

public class ParkingLot {
    private Level[] levels;
    private final int NUM_LEVELS = 5;

    public ParkingLot() {
        levels = new Level[NUM_LEVELS];
        for (int i = 0; i < NUM_LEVELS; i++) {
            levels[i] = new Level(i, 30);
        }
    }

    /* Park the vehicle in a spot (or multiple spots). Return false if failed. */
    public boolean parkVehicle(Vehicle vehicle) {
        for (int i = 0; i < levels.length; i++) {
            if (levels[i].parkVehicle(vehicle)) {
                return true;
            }
        }
        return false;
    }

    public void print() {
        for (int i = 0; i < levels.length; i++) {
            System.out.print("Level" + i + ": ");
            levels[i].print();
            System.out.println("");
        }
        System.out.println("");
    }
}

Level.java

/* Represents a level in a parking garage */
public class Level {
    private int floor;
    private ParkingSpot[] spots;
    private int availableSpots = 0; // number of free spots
    private static final int SPOTS_PER_ROW = 10;

    public Level(int flr, int numberSpots) {
        floor = flr;
        spots = new ParkingSpot[numberSpots];
        int largeSpots = numberSpots / 4;
        int bikeSpots = numberSpots / 4;
        int compactSpots = numberSpots - largeSpots - bikeSpots;
        for (int i = 0; i < numberSpots; i++) {
            VehicleSize sz = VehicleSize.Motorcycle;
            if (i < largeSpots) {
                sz = VehicleSize.Large;
            } else if (i < largeSpots + compactSpots) {
                sz = VehicleSize.Compact;
            }
            int row = i / SPOTS_PER_ROW;
            spots[i] = new ParkingSpot(this, row, i, sz);
        }
        availableSpots = numberSpots;
    }

    public int availableSpots() {
        return availableSpots;
    }

    /* Try to find a place to park this vehicle. Return false if failed. */
    public boolean parkVehicle(Vehicle vehicle) {
        if (availableSpots() < vehicle.getSpotsNeeded()) {
            return false;
        }
        int spotNumber = findAvailableSpots(vehicle);
        if (spotNumber < 0) {
            return false;
        }
        return parkStartingAtSpot(spotNumber, vehicle);
    }

    /* Park a vehicle starting at the spot spotNumber, and continuing until vehicle.spotsNeeded. */
    private boolean parkStartingAtSpot(int spotNumber, Vehicle vehicle) {
        vehicle.clearSpots();
        boolean success = true;
        for (int i = spotNumber; i < spotNumber + vehicle.spotsNeeded; i++) {
             success &= spots[i].park(vehicle);
        }
        availableSpots -= vehicle.spotsNeeded;
        return success;
    }

    /* find a spot to park this vehicle. Return index of spot, or -1 on failure. */
    private int findAvailableSpots(Vehicle vehicle) {
        int spotsNeeded = vehicle.getSpotsNeeded();
        int lastRow = -1;
        int spotsFound = 0;
        for (int i = 0; i < spots.length; i++) {
            ParkingSpot spot = spots[i];
            if (lastRow != spot.getRow()) {
                spotsFound = 0;
                lastRow = spot.getRow();
            }
            if (spot.canFitVehicle(vehicle)) {
                spotsFound++;
            } else {
                spotsFound = 0;
            }
            if (spotsFound == spotsNeeded) {
                return i - (spotsNeeded - 1);
            }
        }
        return -1;
    }

    public void print() {
        int lastRow = -1;
        for (int i = 0; i < spots.length; i++) {
            ParkingSpot spot = spots[i];
            if (spot.getRow() != lastRow) {
                System.out.print("  ");
                lastRow = spot.getRow();
            }
            spot.print();
        }
    }

    /* When a car was removed from the spot, increment availableSpots */
    public void spotFreed() {
        availableSpots++;
    }
}