OOPS! Deep Dive 2: Encapsulation, this & static

// Deep Dive 2: private fields, checked in the constructor, read through getters.
public class Vehicle {
    private final String plate;   // set once, in the constructor
    private final String type;

    public Vehicle(String plate, String type) {
        if (plate == null || plate.isBlank())
            throw new IllegalArgumentException("plate is required");
        this.plate = plate;
        this.type = type;
    }

    public Vehicle(String plate) {
        this(plate, "CAR");
    }

    public String getPlate() { return plate; }   // a getter, and no setter
    public String getType() { return type; }

    public void describe() {
        System.out.println(type + " " + plate);
    }
}

public class ParkingSpot {
    private final int number;
    private Vehicle parked;   // null while free

    public ParkingSpot(int number) {
        if (number < 1)
            throw new IllegalArgumentException("spot number must be 1 or more");
        this.number = number;
    }

    public int getNumber() { return number; }

    public boolean isFree() { return parked == null; }

    public Ticket park(Vehicle v, int hour) {
        if (!isFree())
            throw new IllegalStateException("spot " + number + " is taken");
        parked = v;
        return new Ticket(v, this, hour);   // this = this spot
    }

    public Vehicle leave() {
        Vehicle v = parked;   // null if the spot was already free: no crash
        parked = null;
        return v;
    }
}

public class Ticket {
    private static int nextId = 1;        // one counter, shared by all tickets
    private static final int RATE = 20;   // per started hour

    private final int id;
    private final String plate;
    private final int spotNumber;
    private final int entryHour;

    public Ticket(Vehicle v, ParkingSpot s, int entryHour) {
        this.id = nextId++;
        this.plate = v.getPlate();
        this.spotNumber = s.getNumber();
        this.entryHour = entryHour;
    }

    public static int issued() { return nextId - 1; }

    public int fee(int exitHour) {
        return Math.max(1, exitHour - entryHour) * RATE;
    }

    public void print() {
        System.out.println("Ticket #" + id + " | " + plate + " | spot "
                + spotNumber + " | in at " + entryHour + ":00");
    }
}

public class Main {
    public static void main(String[] args) {
        Vehicle car  = new Vehicle("ABC-1234", "CAR");
        Vehicle bike = new Vehicle("XYZ-77", "BIKE");
        ParkingSpot spot1 = new ParkingSpot(1);
        ParkingSpot spot2 = new ParkingSpot(2);

        Ticket t1 = spot1.park(car, 9);
        Ticket t2 = spot2.park(bike, 10);
        t1.print();
        t2.print();
        System.out.println("Tickets issued: " + Ticket.issued());
        System.out.println("Fee for ticket 1 at 12:00 = " + t1.fee(12));
    }
}

Ticket #1 | ABC-1234 | spot 1 | in at 9:00
Ticket #2 | XYZ-77 | spot 2 | in at 10:00
Tickets issued: 2
Fee for ticket 1 at 12:00 = 60

// Deep Dive 2: private fields, checked in the constructor, read through getters.
public class Vehicle
{
    private readonly string plate;   // set once, in the constructor
    private readonly string type;

    public Vehicle(string plate, string type)
    {
        if (string.IsNullOrWhiteSpace(plate))
            throw new ArgumentException("plate is required");
        this.plate = plate;
        this.type = type;
    }

    public Vehicle(string plate) : this(plate, "CAR")
    {
    }

    public string Plate => plate;   // a getter, and no setter
    public string Type => type;

    public void Describe()
    {
        Console.WriteLine(type + " " + plate);
    }
}

public class ParkingSpot
{
    private readonly int number;
    private Vehicle parked;   // null while free

    public ParkingSpot(int number)
    {
        if (number < 1)
            throw new ArgumentException("spot number must be 1 or more");
        this.number = number;
    }

    public int Number => number;

    public bool IsFree() => parked == null;

    public Ticket Park(Vehicle v, int hour)
    {
        if (!IsFree())
            throw new InvalidOperationException("spot " + number + " is taken");
        parked = v;
        return new Ticket(v, this, hour);   // this = this spot
    }

    public Vehicle Leave()
    {
        Vehicle v = parked;   // null if the spot was already free: no crash
        parked = null;
        return v;
    }
}

public class Ticket
{
    private static int nextId = 1;   // one counter, shared by all tickets
    private const int Rate = 20;     // per started hour (const is always static)

    private readonly int id;
    private readonly string plate;
    private readonly int spotNumber;
    private readonly int entryHour;

    public Ticket(Vehicle v, ParkingSpot s, int entryHour)
    {
        this.id = nextId++;
        this.plate = v.Plate;
        this.spotNumber = s.Number;
        this.entryHour = entryHour;
    }

    public static int Issued() => nextId - 1;

    public int Fee(int exitHour) => Math.Max(1, exitHour - entryHour) * Rate;

    public void Print()
    {
        Console.WriteLine("Ticket #" + id + " | " + plate + " | spot "
            + spotNumber + " | in at " + entryHour + ":00");
    }
}

public class Program
{
    public static void Main(string[] args)
    {
        Vehicle car  = new Vehicle("ABC-1234", "CAR");
        Vehicle bike = new Vehicle("XYZ-77", "BIKE");
        ParkingSpot spot1 = new ParkingSpot(1);
        ParkingSpot spot2 = new ParkingSpot(2);

        Ticket t1 = spot1.Park(car, 9);
        Ticket t2 = spot2.Park(bike, 10);
        t1.Print();
        t2.Print();
        Console.WriteLine("Tickets issued: " + Ticket.Issued());
        Console.WriteLine("Fee for ticket 1 at 12:00 = " + t1.Fee(12));
    }
}

Ticket #1 | ABC-1234 | spot 1 | in at 9:00
Ticket #2 | XYZ-77 | spot 2 | in at 10:00
Tickets issued: 2
Fee for ticket 1 at 12:00 = 60

Practice (Deep Dive 2): bikes pay less than cars.

1. Add a private final String type, copied from the vehicle in the constructor.
2. Replace RATE with two static constants: CAR_RATE = 20 and BIKE_RATE = 10.
3. Make fee() pick the rate that matches the type.
Bonus: in the constructor, reject an entryHour outside 0 to 23
       (throw new IllegalArgumentException("hour must be 0 to 23")).

// Practice (Deep Dive 2): bikes pay less than cars.
//
// 1. Add a private final String type, copied from the vehicle in the constructor.
// 2. Replace RATE with two static constants: CAR_RATE = 20 and BIKE_RATE = 10.
// 3. Make fee() pick the rate that matches the type.
// Bonus: in the constructor, reject an entryHour outside 0 to 23
//        (throw new IllegalArgumentException("hour must be 0 to 23")).
public class Ticket {
    private static int nextId = 1;        // one counter, shared by all tickets
    private static final int RATE = 20;   // per started hour

    private final int id;
    private final String plate;
    private final int spotNumber;
    private final int entryHour;

    public Ticket(Vehicle v, ParkingSpot s, int entryHour) {
        this.id = nextId++;
        this.plate = v.getPlate();
        this.spotNumber = s.getNumber();
        this.entryHour = entryHour;
    }

    public static int issued() { return nextId - 1; }

    public int fee(int exitHour) {
        return Math.max(1, exitHour - entryHour) * RATE;
    }

    public void print() {
        System.out.println("Ticket #" + id + " | " + plate + " | spot "
                + spotNumber + " | in at " + entryHour + ":00");
    }
}

public class Main {
    public static void main(String[] args) {
        Vehicle car  = new Vehicle("ABC-1234", "CAR");
        Vehicle bike = new Vehicle("XYZ-77", "BIKE");
        ParkingSpot spot1 = new ParkingSpot(1);
        ParkingSpot spot2 = new ParkingSpot(2);

        Ticket t1 = spot1.park(car, 9);
        Ticket t2 = spot2.park(bike, 10);
        t1.print();
        t2.print();
        System.out.println("Tickets issued: " + Ticket.issued());
        System.out.println("Car fee at 12:00 = " + t1.fee(12));
        System.out.println("Bike fee at 12:00 = " + t2.fee(12));
    }
}

// Practice (Deep Dive 2): bikes pay less than cars.
//
// 1. Add a private readonly string type, copied from the vehicle in the constructor.
// 2. Replace Rate with two constants: CarRate = 20 and BikeRate = 10.
// 3. Make Fee() pick the rate that matches the type.
// Bonus: in the constructor, reject an entryHour outside 0 to 23
//        (throw new ArgumentException("hour must be 0 to 23")).
public class Ticket
{
    private static int nextId = 1;   // one counter, shared by all tickets
    private const int Rate = 20;     // per started hour (const is always static)

    private readonly int id;
    private readonly string plate;
    private readonly int spotNumber;
    private readonly int entryHour;

    public Ticket(Vehicle v, ParkingSpot s, int entryHour)
    {
        this.id = nextId++;
        this.plate = v.Plate;
        this.spotNumber = s.Number;
        this.entryHour = entryHour;
    }

    public static int Issued() => nextId - 1;

    public int Fee(int exitHour) => Math.Max(1, exitHour - entryHour) * Rate;

    public void Print()
    {
        Console.WriteLine("Ticket #" + id + " | " + plate + " | spot "
            + spotNumber + " | in at " + entryHour + ":00");
    }
}

public class Program
{
    public static void Main(string[] args)
    {
        Vehicle car  = new Vehicle("ABC-1234", "CAR");
        Vehicle bike = new Vehicle("XYZ-77", "BIKE");
        ParkingSpot spot1 = new ParkingSpot(1);
        ParkingSpot spot2 = new ParkingSpot(2);

        Ticket t1 = spot1.Park(car, 9);
        Ticket t2 = spot2.Park(bike, 10);
        t1.Print();
        t2.Print();
        Console.WriteLine("Tickets issued: " + Ticket.Issued());
        Console.WriteLine("Car fee at 12:00 = " + t1.Fee(12));
        Console.WriteLine("Bike fee at 12:00 = " + t2.Fee(12));
    }
}