OOPS! Deep Dive 5: Composition & the Full System

// Deep Dive 5: unchanged since Deep Dive 4. Vehicle is abstract; each kind brings its own rate and 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;   // has-a Vehicle
    private final ParkingSpot spot;  // has-a ParkingSpot
    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.spot = s;
        this.entryHour = entryHour;
    }

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

    public ParkingSpot getSpot() { return spot; }

    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 "
                + spot.getNumber() + " | 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;
    }
}

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

// Deep Dive 5: composition. The lot is made of spots and keeps the open tickets.
public class ParkingLot {
    private final List<ParkingSpot> spots = new ArrayList<>();   // the lot owns its spots
    private final Map<String, Ticket> open = new HashMap<>();    // plate -> ticket

    public ParkingLot(int size) {
        if (size < 1)
            throw new IllegalArgumentException("a lot needs at least 1 spot");
        for (int i = 1; i <= size; i++)
            spots.add(new ParkingSpot(i));   // made here: they live and die with the lot
    }

    public Ticket enter(Vehicle v, int hour) {
        for (ParkingSpot s : spots) {
            if (s.isFree()) {
                Ticket t = s.park(v, hour);
                open.put(v.getPlate(), t);
                return t;
            }
        }
        throw new IllegalStateException("lot is full");
    }

    public int exit(String plate, int hour) {
        Ticket t = open.remove(plate);
        if (t == null)
            throw new IllegalArgumentException("no vehicle with plate " + plate);
        t.getSpot().leave();   // free the spot again
        return t.fee(hour);
    }

    public int freeSpots() {
        int free = 0;
        for (ParkingSpot s : spots)
            if (s.isFree()) free++;
        return free;
    }
}

public class Truck extends Vehicle {
    private final int axles;

    public Truck(String plate, int axles) {
        super(plate);
        if (axles < 2)
            throw new IllegalArgumentException("a truck has at least 2 axles");
        this.axles = axles;
    }

    public int getAxles() { return axles; }

    @Override
    public int hourlyRate() { return 40; }

    @Override
    public int fee(int hours) {
        return super.fee(Math.max(2, hours));   // at least 2 hours
    }
}

public class Main {
    public static void main(String[] args) {
        ParkingLot lot = new ParkingLot(4);

        lot.enter(new Car("ABC-1234"), 9).print();
        lot.enter(new Bike("XYZ-77"), 10).print();
        lot.enter(new ElectricCar("EVX-2025"), 11).print();
        lot.enter(new Truck("TRK-4040", 3), 11).print();
        System.out.println("Free spots: " + lot.freeSpots());

        try {
            lot.enter(new Car("NEW-5555"), 11);
        } catch (IllegalStateException e) {
            System.out.println("Refused: " + e.getMessage());
        }

        System.out.println("ABC-1234 pays " + lot.exit("ABC-1234", 12));
        lot.enter(new Car("NEW-5555"), 12).print();   // spot 1 is free again
        System.out.println("Free spots: " + lot.freeSpots());
    }
}

Ticket #1 | ABC-1234 | spot 1 | in at 9:00
Ticket #2 | XYZ-77 | spot 2 | in at 10:00
Ticket #3 | EVX-2025 | spot 3 | in at 11:00
Ticket #4 | TRK-4040 | spot 4 | in at 11:00
Free spots: 0
Refused: lot is full
ABC-1234 pays 60
Ticket #5 | NEW-5555 | spot 1 | in at 12:00
Free spots: 0

// Deep Dive 5: unchanged since Deep Dive 4. Vehicle is abstract; each kind brings its own rate and 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;   // has-a Vehicle
    private readonly ParkingSpot spot;  // has-a ParkingSpot
    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.spot = s;
        this.entryHour = entryHour;
    }

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

    public ParkingSpot Spot => spot;

    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 "
            + spot.Number + " | 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);
}

// Deep Dive 5: composition. The lot is made of spots and keeps the open tickets.
public class ParkingLot
{
    private readonly List<ParkingSpot> spots = new();            // the lot owns its spots
    private readonly Dictionary<string, Ticket> open = new();    // plate -> ticket

    public ParkingLot(int size)
    {
        if (size < 1)
            throw new ArgumentException("a lot needs at least 1 spot");
        for (int i = 1; i <= size; i++)
            spots.Add(new ParkingSpot(i));   // made here: they live and die with the lot
    }

    public Ticket Enter(Vehicle v, int hour)
    {
        foreach (ParkingSpot s in spots)
        {
            if (s.IsFree())
            {
                Ticket t = s.Park(v, hour);
                open[v.Plate] = t;
                return t;
            }
        }
        throw new InvalidOperationException("lot is full");
    }

    public int Exit(string plate, int hour)
    {
        if (!open.Remove(plate, out Ticket t))
            throw new ArgumentException("no vehicle with plate " + plate);
        t.Spot.Leave();   // free the spot again
        return t.Fee(hour);
    }

    public int FreeSpots()
    {
        int free = 0;
        foreach (ParkingSpot s in spots)
            if (s.IsFree()) free++;
        return free;
    }
}

public class Truck : Vehicle
{
    private readonly int axles;

    public Truck(string plate, int axles) : base(plate)
    {
        if (axles < 2)
            throw new ArgumentException("a truck has at least 2 axles");
        this.axles = axles;
    }

    public int Axles => axles;

    public override int HourlyRate() => 40;

    public override int Fee(int hours)
    {
        return base.Fee(Math.Max(2, hours));   // at least 2 hours
    }
}

public class Program
{
    public static void Main(string[] args)
    {
        ParkingLot lot = new ParkingLot(4);

        lot.Enter(new Car("ABC-1234"), 9).Print();
        lot.Enter(new Bike("XYZ-77"), 10).Print();
        lot.Enter(new ElectricCar("EVX-2025"), 11).Print();
        lot.Enter(new Truck("TRK-4040", 3), 11).Print();
        Console.WriteLine("Free spots: " + lot.FreeSpots());

        try
        {
            lot.Enter(new Car("NEW-5555"), 11);
        }
        catch (InvalidOperationException e)
        {
            Console.WriteLine("Refused: " + e.Message);
        }

        Console.WriteLine("ABC-1234 pays " + lot.Exit("ABC-1234", 12));
        lot.Enter(new Car("NEW-5555"), 12).Print();   // spot 1 is free again
        Console.WriteLine("Free spots: " + lot.FreeSpots());
    }
}

Ticket #1 | ABC-1234 | spot 1 | in at 9:00
Ticket #2 | XYZ-77 | spot 2 | in at 10:00
Ticket #3 | EVX-2025 | spot 3 | in at 11:00
Ticket #4 | TRK-4040 | spot 4 | in at 11:00
Free spots: 0
Refused: lot is full
ABC-1234 pays 60
Ticket #5 | NEW-5555 | spot 1 | in at 12:00
Free spots: 0

Practice (Deep Dive 5): the owner wants to know what the lot earned today.

1. Add a private int field earned to ParkingLot.
2. In exit(), add each fee to earned before returning it.
3. Add a getter earned() that returns the total.
Bonus: in enter(), refuse a plate that is already inside
       (throw new IllegalStateException(plate + " is already parked")).

Then uncomment the lines at the end of Main.java.
Expected: XYZ-77 pays 30, then Earned today = 90

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

// Practice (Deep Dive 5): the owner wants to know what the lot earned today.
//
// 1. Add a private int field earned to ParkingLot.
// 2. In exit(), add each fee to earned before returning it.
// 3. Add a getter earned() that returns the total.
// Bonus: in enter(), refuse a plate that is already inside
//        (throw new IllegalStateException(plate + " is already parked")).
//
// Then uncomment the lines at the end of Main.java.
// Expected: XYZ-77 pays 30, then Earned today = 90
public class ParkingLot {
    private final List<ParkingSpot> spots = new ArrayList<>();   // the lot owns its spots
    private final Map<String, Ticket> open = new HashMap<>();    // plate -> ticket

    public ParkingLot(int size) {
        if (size < 1)
            throw new IllegalArgumentException("a lot needs at least 1 spot");
        for (int i = 1; i <= size; i++)
            spots.add(new ParkingSpot(i));   // made here: they live and die with the lot
    }

    public Ticket enter(Vehicle v, int hour) {
        for (ParkingSpot s : spots) {
            if (s.isFree()) {
                Ticket t = s.park(v, hour);
                open.put(v.getPlate(), t);
                return t;
            }
        }
        throw new IllegalStateException("lot is full");
    }

    public int exit(String plate, int hour) {
        Ticket t = open.remove(plate);
        if (t == null)
            throw new IllegalArgumentException("no vehicle with plate " + plate);
        t.getSpot().leave();   // free the spot again
        return t.fee(hour);
    }

    public int freeSpots() {
        int free = 0;
        for (ParkingSpot s : spots)
            if (s.isFree()) free++;
        return free;
    }
}

public class Main {
    public static void main(String[] args) {
        ParkingLot lot = new ParkingLot(4);

        lot.enter(new Car("ABC-1234"), 9).print();
        lot.enter(new Bike("XYZ-77"), 10).print();
        lot.enter(new ElectricCar("EVX-2025"), 11).print();
        lot.enter(new Truck("TRK-4040", 3), 11).print();
        System.out.println("Free spots: " + lot.freeSpots());

        try {
            lot.enter(new Car("NEW-5555"), 11);
        } catch (IllegalStateException e) {
            System.out.println("Refused: " + e.getMessage());
        }

        System.out.println("ABC-1234 pays " + lot.exit("ABC-1234", 12));
        lot.enter(new Car("NEW-5555"), 12).print();   // spot 1 is free again
        System.out.println("Free spots: " + lot.freeSpots());

        // When earned() is done, uncomment these lines:
        // System.out.println("XYZ-77 pays " + lot.exit("XYZ-77", 13));
        // System.out.println("Earned today = " + lot.earned());
    }
}

// Practice (Deep Dive 5): the owner wants to know what the lot earned today.
//
// 1. Add a private int field earned to ParkingLot.
// 2. In Exit(), add each fee to earned before returning it.
// 3. Add a read-only property Earned that returns the total.
// Bonus: in Enter(), refuse a plate that is already inside
//        (throw new InvalidOperationException(plate + " is already parked")).
//
// Then uncomment the lines at the end of Program.cs.
// Expected: XYZ-77 pays 30, then Earned today = 90
public class ParkingLot
{
    private readonly List<ParkingSpot> spots = new();            // the lot owns its spots
    private readonly Dictionary<string, Ticket> open = new();    // plate -> ticket

    public ParkingLot(int size)
    {
        if (size < 1)
            throw new ArgumentException("a lot needs at least 1 spot");
        for (int i = 1; i <= size; i++)
            spots.Add(new ParkingSpot(i));   // made here: they live and die with the lot
    }

    public Ticket Enter(Vehicle v, int hour)
    {
        foreach (ParkingSpot s in spots)
        {
            if (s.IsFree())
            {
                Ticket t = s.Park(v, hour);
                open[v.Plate] = t;
                return t;
            }
        }
        throw new InvalidOperationException("lot is full");
    }

    public int Exit(string plate, int hour)
    {
        if (!open.Remove(plate, out Ticket t))
            throw new ArgumentException("no vehicle with plate " + plate);
        t.Spot.Leave();   // free the spot again
        return t.Fee(hour);
    }

    public int FreeSpots()
    {
        int free = 0;
        foreach (ParkingSpot s in spots)
            if (s.IsFree()) free++;
        return free;
    }
}

public class Program
{
    public static void Main(string[] args)
    {
        ParkingLot lot = new ParkingLot(4);

        lot.Enter(new Car("ABC-1234"), 9).Print();
        lot.Enter(new Bike("XYZ-77"), 10).Print();
        lot.Enter(new ElectricCar("EVX-2025"), 11).Print();
        lot.Enter(new Truck("TRK-4040", 3), 11).Print();
        Console.WriteLine("Free spots: " + lot.FreeSpots());

        try
        {
            lot.Enter(new Car("NEW-5555"), 11);
        }
        catch (InvalidOperationException e)
        {
            Console.WriteLine("Refused: " + e.Message);
        }

        Console.WriteLine("ABC-1234 pays " + lot.Exit("ABC-1234", 12));
        lot.Enter(new Car("NEW-5555"), 12).Print();   // spot 1 is free again
        Console.WriteLine("Free spots: " + lot.FreeSpots());

        // When Earned is done, uncomment these lines:
        // Console.WriteLine("XYZ-77 pays " + lot.Exit("XYZ-77", 13));
        // Console.WriteLine("Earned today = " + lot.Earned);
    }
}