OOPS! Deep Dive 4: Polymorphism, Abstraction & Interfaces

// Deep Dive 4: Vehicle is abstract. Every vehicle must say its rate, and can change the fee rule.
public abstract class Vehicle {
    private final String plate;

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

    public String getPlate() { return plate; }

    public abstract int hourlyRate();   // no body: each kind must fill it in

    public int fee(int hours) {          // the usual rule; a child may override it
        return hours * 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 Vehicle vehicle;
    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.vehicle = v;
        this.spotNumber = s.getNumber();
        this.entryHour = entryHour;
    }

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

    public int fee(int exitHour) {
        int hours = Math.max(1, exitHour - entryHour);
        return vehicle.fee(hours);   // polymorphism: the vehicle's own rule runs
    }

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

public class Bike extends Vehicle {
    public Bike(String plate) {
        super(plate);
    }

    @Override
    public int hourlyRate() { return 10; }
}

public class Car extends Vehicle {
    public Car(String plate) {
        super(plate);
    }

    @Override
    public int hourlyRate() { return 20; }
}

// An interface: a promise of what a class can do, not what it is.
public interface Chargeable {
    int chargingFee(int kwh);
}

public class ChargingPoint {
    // Takes anything Chargeable: it doesn't care what the thing is.
    public void charge(Chargeable c, int kwh) {
        System.out.println("Charging " + kwh + " kWh = " + c.chargingFee(kwh));
    }
}

public class ElectricCar extends Car implements Chargeable {
    public ElectricCar(String plate) {
        super(plate);
    }

    @Override
    public int fee(int hours) {
        return super.fee(hours) / 2;   // half price: a different rule
    }

    @Override
    public int chargingFee(int kwh) {
        return kwh * 3;
    }
}

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

        Ticket t1 = spot1.park(car, 9);
        Ticket t2 = spot2.park(bike, 10);
        Ticket t3 = spot3.park(ev, 11);

        Vehicle[] all = { car, bike, ev };
        for (Vehicle v : all)
            v.describe();   // one call, three classes

        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));

        ChargingPoint charger = new ChargingPoint();
        charger.charge(ev, 10);
    }
}

Car ABC-1234
Bike XYZ-77
ElectricCar EVX-2025
Car fee at 12:00 = 60
Bike fee at 12:00 = 20
EV fee at 12:00 = 10
Charging 10 kWh = 30

// Deep Dive 4: Vehicle is abstract. Every vehicle must say its rate, and can change the fee rule.
public abstract class Vehicle
{
    private readonly string plate;

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

    public string Plate => plate;

    public abstract int HourlyRate();   // no body: each kind must fill it in

    public virtual int Fee(int hours)   // the usual rule; virtual: a child may override it
    {
        return hours * 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 Vehicle vehicle;
    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.vehicle = v;
        this.spotNumber = s.Number;
        this.entryHour = entryHour;
    }

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

    public int Fee(int exitHour)
    {
        int hours = Math.Max(1, exitHour - entryHour);
        return vehicle.Fee(hours);   // polymorphism: the vehicle's own rule runs
    }

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

public class Bike : Vehicle
{
    public Bike(string plate) : base(plate)
    {
    }

    public override int HourlyRate() => 10;
}

public class Car : Vehicle
{
    public Car(string plate) : base(plate)
    {
    }

    public override int HourlyRate() => 20;
}

public class ChargingPoint
{
    // Takes anything IChargeable: it doesn't care what the thing is.
    public void Charge(IChargeable c, int kwh)
    {
        Console.WriteLine("Charging " + kwh + " kWh = " + c.ChargingFee(kwh));
    }
}

public class ElectricCar : Car, IChargeable
{
    public ElectricCar(string plate) : base(plate)
    {
    }

    public override int Fee(int hours)
    {
        return base.Fee(hours) / 2;   // half price: a different rule
    }

    public int ChargingFee(int kwh) => kwh * 3;
}

// An interface: a promise of what a class can do, not what it is.
public interface IChargeable
{
    int ChargingFee(int kwh);
}

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

        Ticket t1 = spot1.Park(car, 9);
        Ticket t2 = spot2.Park(bike, 10);
        Ticket t3 = spot3.Park(ev, 11);

        Vehicle[] all = { car, bike, ev };
        foreach (Vehicle v in all)
            v.Describe();   // one call, three classes

        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));

        ChargingPoint charger = new ChargingPoint();
        charger.Charge(ev, 10);
    }
}

Car ABC-1234
Bike XYZ-77
ElectricCar EVX-2025
Car fee at 12:00 = 60
Bike fee at 12:00 = 20
EV fee at 12:00 = 10
Charging 10 kWh = 30

Practice (Deep Dive 4): bring the Truck from Deep Dive 3 into the new design.

1. Write a class Truck that extends Vehicle, with its own field: int axles.
   Constructor: Truck(String plate, int axles). Call super(plate) first,
   and reject fewer than 2 axles.
2. Vehicle now asks every kind for its rate: override hourlyRate() and return 40.
3. Trucks pay for at least 2 hours: override fee(int hours).
Bonus: reuse the parent's rule with super.fee(...) instead of writing hours * rate again.

Ticket must not change. Then uncomment the truck lines in Main.java.
Expected: Truck fee at 12:00 = 80   (in at 11: 1 hour, charged as 2)

// Practice (Deep Dive 4): bring the Truck from Deep Dive 3 into the new design.
//
// 1. Write a class Truck that extends Vehicle, with its own field: int axles.
//    Constructor: Truck(String plate, int axles). Call super(plate) first,
//    and reject fewer than 2 axles.
// 2. Vehicle now asks every kind for its rate: override hourlyRate() and return 40.
// 3. Trucks pay for at least 2 hours: override fee(int hours).
// Bonus: reuse the parent's rule with super.fee(...) instead of writing hours * rate again.
//
// Ticket must not change. Then uncomment the truck lines in Main.java.
// Expected: Truck fee at 12:00 = 80   (in at 11: 1 hour, charged as 2)

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

        Ticket t1 = spot1.park(car, 9);
        Ticket t2 = spot2.park(bike, 10);
        Ticket t3 = spot3.park(ev, 11);

        Vehicle[] all = { car, bike, ev };
        for (Vehicle v : all)
            v.describe();   // one call, three classes

        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));

        ChargingPoint charger = new ChargingPoint();
        charger.charge(ev, 10);

        // When Truck is done, uncomment these lines:
        // ParkingSpot spot4 = new ParkingSpot(4);
        // Truck truck = new Truck("TRK-4040", 3);
        // Ticket t4 = spot4.park(truck, 11);
        // truck.describe();
        // System.out.println("Truck fee at 12:00 = " + t4.fee(12));
    }
}

// Practice (Deep Dive 4): bring the Truck from Deep Dive 3 into the new design.
//
// 1. Write a class Truck : Vehicle, with its own field: int axles.
//    Constructor: Truck(string plate, int axles) : base(plate),
//    and reject fewer than 2 axles.
// 2. Vehicle now asks every kind for its rate: override HourlyRate() and return 40.
// 3. Trucks pay for at least 2 hours: override Fee(int hours).
// Bonus: reuse the parent's rule with base.Fee(...) instead of writing hours * rate again.
//
// Ticket must not change. Then uncomment the truck lines in Program.cs.
// Expected: Truck fee at 12:00 = 80   (in at 11: 1 hour, charged as 2)

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

        Ticket t1 = spot1.Park(car, 9);
        Ticket t2 = spot2.Park(bike, 10);
        Ticket t3 = spot3.Park(ev, 11);

        Vehicle[] all = { car, bike, ev };
        foreach (Vehicle v in all)
            v.Describe();   // one call, three classes

        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));

        ChargingPoint charger = new ChargingPoint();
        charger.Charge(ev, 10);

        // When Truck is done, uncomment these lines:
        // ParkingSpot spot4 = new ParkingSpot(4);
        // Truck truck = new Truck("TRK-4040", 3);
        // Ticket t4 = spot4.Park(truck, 11);
        // truck.Describe();
        // Console.WriteLine("Truck fee at 12:00 = " + t4.Fee(12));
    }
}