OOPS! Deep Dive 3: Inheritance (super / base)

// Deep Dive 3: Vehicle is now the parent class. Car and Bike extend it.
public class Vehicle {
    private final String plate;
    protected final int hourlyRate;   // protected: child classes can read it

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

    public String getPlate() { return plate; }
    public int getHourlyRate() { return hourlyRate; }

    public void describe() {
        System.out.println(getClass().getSimpleName() + " " + 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 final int id;
    private final String plate;
    private final int rate;
    private final int spotNumber;
    private final int entryHour;

    public Ticket(Vehicle v, ParkingSpot s, int entryHour) {
        if (entryHour < 0 || entryHour > 23)
            throw new IllegalArgumentException("hour must be 0 to 23");
        this.id = nextId++;
        this.plate = v.getPlate();
        this.rate = v.getHourlyRate();   // no type check: the vehicle knows its rate
        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 Bike extends Vehicle {
    public Bike(String plate) {
        super(plate, 10);
    }
}

public class Car extends Vehicle {
    private final boolean electric;

    public Car(String plate, boolean electric) {
        super(plate, 20);   // build the parent part first
        this.electric = electric;
    }

    public Car(String plate) {
        this(plate, false);
    }

    public boolean isElectric() { return electric; }
}

public class Main {
    public static void main(String[] args) {
        Car car   = new Car("ABC-1234");
        Bike bike = new Bike("XYZ-77");
        Car ev    = new Car("EVX-2025", true);
        ParkingSpot spot1 = new ParkingSpot(1);
        ParkingSpot spot2 = new ParkingSpot(2);
        ParkingSpot spot3 = new ParkingSpot(3);

        Ticket t1 = spot1.park(car, 9);    // a Car IS-A Vehicle
        Ticket t2 = spot2.park(bike, 10);
        Ticket t3 = spot3.park(ev, 11);

        car.describe();
        bike.describe();
        ev.describe();
        System.out.println("Car fee at 12:00 = " + t1.fee(12));
        System.out.println("Bike fee at 12:00 = " + t2.fee(12));
        System.out.println("EV fee at 12:00 = " + t3.fee(12));
    }
}

Car ABC-1234
Bike XYZ-77
Car EVX-2025
Car fee at 12:00 = 60
Bike fee at 12:00 = 20
EV fee at 12:00 = 20

// Deep Dive 3: Vehicle is now the parent class. Car and Bike extend it.
public class Vehicle
{
    private readonly string plate;
    protected readonly int hourlyRate;   // protected: child classes can read it

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

    public string Plate => plate;
    public int HourlyRate => hourlyRate;

    public void Describe()
    {
        Console.WriteLine(GetType().Name + " " + 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 readonly int id;
    private readonly string plate;
    private readonly int rate;
    private readonly int spotNumber;
    private readonly int entryHour;

    public Ticket(Vehicle v, ParkingSpot s, int entryHour)
    {
        if (entryHour < 0 || entryHour > 23)
            throw new ArgumentException("hour must be 0 to 23");
        this.id = nextId++;
        this.plate = v.Plate;
        this.rate = v.HourlyRate;   // no type check: the vehicle knows its rate
        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 Bike : Vehicle
{
    public Bike(string plate) : base(plate, 10)
    {
    }
}

public class Car : Vehicle
{
    private readonly bool electric;

    public Car(string plate, bool electric) : base(plate, 20)   // build the parent part first
    {
        this.electric = electric;
    }

    public Car(string plate) : this(plate, false)
    {
    }

    public bool IsElectric => electric;
}

public class Program
{
    public static void Main(string[] args)
    {
        Car car   = new Car("ABC-1234");
        Bike bike = new Bike("XYZ-77");
        Car ev    = new Car("EVX-2025", true);
        ParkingSpot spot1 = new ParkingSpot(1);
        ParkingSpot spot2 = new ParkingSpot(2);
        ParkingSpot spot3 = new ParkingSpot(3);

        Ticket t1 = spot1.Park(car, 9);    // a Car IS-A Vehicle
        Ticket t2 = spot2.Park(bike, 10);
        Ticket t3 = spot3.Park(ev, 11);

        car.Describe();
        bike.Describe();
        ev.Describe();
        Console.WriteLine("Car fee at 12:00 = " + t1.Fee(12));
        Console.WriteLine("Bike fee at 12:00 = " + t2.Fee(12));
        Console.WriteLine("EV fee at 12:00 = " + t3.Fee(12));
    }
}

Car ABC-1234
Bike XYZ-77
Car EVX-2025
Car fee at 12:00 = 60
Bike fee at 12:00 = 20
EV fee at 12:00 = 20

Practice (Deep Dive 3): trucks are coming to the lot.

1. Write a class Truck that extends Vehicle, at 40 per hour.
2. Give it its own field: int axles.
3. Constructor: Truck(String plate, int axles). Call the parent constructor first.
4. Add getAxles().
Bonus: reject fewer than 2 axles
       (throw new IllegalArgumentException("a truck has at least 2 axles")).

Then uncomment the truck lines in Main.java. Expected: Truck fee at 12:00 = 120

// Practice (Deep Dive 3): trucks are coming to the lot.
//
// 1. Write a class Truck that extends Vehicle, at 40 per hour.
// 2. Give it its own field: int axles.
// 3. Constructor: Truck(String plate, int axles). Call the parent constructor first.
// 4. Add getAxles().
// Bonus: reject fewer than 2 axles
//        (throw new IllegalArgumentException("a truck has at least 2 axles")).
//
// Then uncomment the truck lines in Main.java. Expected: Truck fee at 12:00 = 120

public class Main {
    public static void main(String[] args) {
        Car car   = new Car("ABC-1234");
        Bike bike = new Bike("XYZ-77");
        Car ev    = new Car("EVX-2025", true);
        ParkingSpot spot1 = new ParkingSpot(1);
        ParkingSpot spot2 = new ParkingSpot(2);
        ParkingSpot spot3 = new ParkingSpot(3);

        Ticket t1 = spot1.park(car, 9);    // a Car IS-A Vehicle
        Ticket t2 = spot2.park(bike, 10);
        Ticket t3 = spot3.park(ev, 11);

        car.describe();
        bike.describe();
        ev.describe();
        System.out.println("Car fee at 12:00 = " + t1.fee(12));
        System.out.println("Bike fee at 12:00 = " + t2.fee(12));
        System.out.println("EV fee at 12:00 = " + t3.fee(12));

        // When Truck is done, uncomment these lines:
        // ParkingSpot spot4 = new ParkingSpot(4);
        // Truck truck = new Truck("TRK-4040", 3);
        // Ticket t4 = spot4.park(truck, 9);
        // truck.describe();
        // System.out.println("Truck fee at 12:00 = " + t4.fee(12));
    }
}

// Practice (Deep Dive 3): trucks are coming to the lot.
//
// 1. Write a class Truck that extends Vehicle (Truck : Vehicle), at 40 per hour.
// 2. Give it its own field: int axles.
// 3. Constructor: Truck(string plate, int axles). Call the parent constructor first (: base(...)).
// 4. Add a read-only property Axles.
// Bonus: reject fewer than 2 axles
//        (throw new ArgumentException("a truck has at least 2 axles")).
//
// Then uncomment the truck lines in Program.cs. Expected: Truck fee at 12:00 = 120

public class Program
{
    public static void Main(string[] args)
    {
        Car car   = new Car("ABC-1234");
        Bike bike = new Bike("XYZ-77");
        Car ev    = new Car("EVX-2025", true);
        ParkingSpot spot1 = new ParkingSpot(1);
        ParkingSpot spot2 = new ParkingSpot(2);
        ParkingSpot spot3 = new ParkingSpot(3);

        Ticket t1 = spot1.Park(car, 9);    // a Car IS-A Vehicle
        Ticket t2 = spot2.Park(bike, 10);
        Ticket t3 = spot3.Park(ev, 11);

        car.Describe();
        bike.Describe();
        ev.Describe();
        Console.WriteLine("Car fee at 12:00 = " + t1.Fee(12));
        Console.WriteLine("Bike fee at 12:00 = " + t2.Fee(12));
        Console.WriteLine("EV fee at 12:00 = " + t3.Fee(12));

        // When Truck is done, uncomment these lines:
        // ParkingSpot spot4 = new ParkingSpot(4);
        // Truck truck = new Truck("TRK-4040", 3);
        // Ticket t4 = spot4.Park(truck, 9);
        // truck.Describe();
        // Console.WriteLine("Truck fee at 12:00 = " + t4.Fee(12));
    }
}