OOPS! Deep Dive 1: Classes, Objects & Constructors

Build a Parking Lot, part 1 of 5. Code and practice for the video, in Java and C#.

What you'll learn

Find the classes in a problem: nouns become classes, details become fields, verbs become methods
Write a class with fields and a constructor, and create objects with new
Overload a constructor and chain it with this(...)
Give one object a reference to another (a ParkingSpot holds a Vehicle)

Download

parking-lot-dd01-java.zip: Java (lesson code, practice starter, solution)
parking-lot-dd01-csharp.zip: C# (the same)

The code from the video

Java

Vehicle.java

// Deep Dive 1: a class is a blueprint, an object is a thing built from it.
public class Vehicle {
    // Fields: what every vehicle knows about itself.
    // (Open to everyone for now; Deep Dive 2 locks them down.)
    String plate;
    String type;   // "CAR" or "BIKE"

    // Constructor: same name as the class, no return type.
    // It runs once, at the moment `new Vehicle(...)` creates the object.
    Vehicle(String plate, String type) {
        this.plate = plate;   // this.plate = the field, plate = the parameter
        this.type = type;
    }

    // Overloaded constructor: only a plate, so assume a car.
    Vehicle(String plate) {
        this(plate, "CAR");   // hand the work to the other constructor
    }

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

ParkingSpot.java

public class ParkingSpot {
    int number;
    Vehicle parked;   // null while the spot is free

    ParkingSpot(int number) {
        this.number = number;
    }

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

    void park(Vehicle v) {
        parked = v;
        System.out.println("Spot " + number + ": parked " + v.plate);
    }

    void leave() {
        System.out.println("Spot " + number + ": " + parked.plate + " left");
        parked = null;
    }
}

Main.java

public class Main {
    public static void main(String[] args) {
        // One blueprint, three separate objects.
        Vehicle car  = new Vehicle("ABC-1234", "CAR");
        Vehicle bike = new Vehicle("XYZ-77", "BIKE");
        Vehicle taxi = new Vehicle("TAX-5555");   // type defaults to CAR

        car.describe();
        bike.describe();
        taxi.describe();

        ParkingSpot spot1 = new ParkingSpot(1);
        ParkingSpot spot2 = new ParkingSpot(2);

        spot1.park(car);
        spot2.park(bike);
        System.out.println("Spot 1 free? " + spot1.isFree());

        spot1.leave();
        System.out.println("Spot 1 free? " + spot1.isFree());
    }
}

Output:

CAR ABC-1234
BIKE XYZ-77
CAR TAX-5555
Spot 1: parked ABC-1234
Spot 2: parked XYZ-77
Spot 1 free? false
Spot 1: ABC-1234 left
Spot 1 free? true

C#

Vehicle.cs

// Deep Dive 1: a class is a blueprint, an object is a thing built from it.
public class Vehicle
{
    // Fields: what every vehicle knows about itself.
    // (Open to everyone for now; Deep Dive 2 locks them down.)
    public string Plate;
    public string Type;   // "CAR" or "BIKE"

    // Constructor: same name as the class, no return type.
    // It runs once, at the moment `new Vehicle(...)` creates the object.
    public Vehicle(string plate, string type)
    {
        this.Plate = plate;   // this.Plate = the field, plate = the parameter
        this.Type = type;
    }

    // Overloaded constructor: only a plate, so assume a car.
    public Vehicle(string plate) : this(plate, "CAR")   // hand the work to the other constructor
    {
    }

    public void Describe()
    {
        Console.WriteLine(Type + " " + Plate);
    }
}

ParkingSpot.cs

public class ParkingSpot
{
    public int Number;
    public Vehicle Parked;   // null while the spot is free

    public ParkingSpot(int number)
    {
        this.Number = number;
    }

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

    public void Park(Vehicle v)
    {
        Parked = v;
        Console.WriteLine("Spot " + Number + ": parked " + v.Plate);
    }

    public void Leave()
    {
        Console.WriteLine("Spot " + Number + ": " + Parked.Plate + " left");
        Parked = null;
    }
}

Program.cs

public class Program
{
    public static void Main(string[] args)
    {
        // One blueprint, three separate objects.
        Vehicle car  = new Vehicle("ABC-1234", "CAR");
        Vehicle bike = new Vehicle("XYZ-77", "BIKE");
        Vehicle taxi = new Vehicle("TAX-5555");   // type defaults to CAR

        car.Describe();
        bike.Describe();
        taxi.Describe();

        ParkingSpot spot1 = new ParkingSpot(1);
        ParkingSpot spot2 = new ParkingSpot(2);

        spot1.Park(car);
        spot2.Park(bike);
        Console.WriteLine("Spot 1 free? " + spot1.IsFree());

        spot1.Leave();
        Console.WriteLine("Spot 1 free? " + spot1.IsFree());
    }
}

Output:

CAR ABC-1234
BIKE XYZ-77
CAR TAX-5555
Spot 1: parked ABC-1234
Spot 2: parked XYZ-77
Spot 1 free? False
Spot 1: ABC-1234 left
Spot 1 free? True

Practice: Build a Ticket class

Practice (Deep Dive 1): build the Ticket class.

A ticket remembers:
  - id          (int)
  - plate       (String)  the vehicle's plate
  - spotNumber  (int)
  - entryHour   (int)     0 to 23

1. Add the four fields.
2. Add a constructor Ticket(int id, Vehicle v, ParkingSpot s, int entryHour)
   that copies the plate from v and the number from s.
3. Add print(), which prints:  Ticket #1 | ABC-1234 | spot 1 | in at 9:00
Bonus: add fee(int exitHour) that returns 20 for every started hour.

(Names above are the Java ones; the C# starter below uses the C# names in its comments.)

Start from the lesson code above. Only these files change; the rest are the same as the lesson.

Starter (Java)

Ticket.java

// Practice (Deep Dive 1): build the Ticket class.
//
// A ticket remembers:
//   - id          (int)
//   - plate       (String)  the vehicle's plate
//   - spotNumber  (int)
//   - entryHour   (int)     0 to 23
//
// 1. Add the four fields.
// 2. Add a constructor Ticket(int id, Vehicle v, ParkingSpot s, int entryHour)
//    that copies the plate from v and the number from s.
// 3. Add print(), which prints:  Ticket #1 | ABC-1234 | spot 1 | in at 9:00
// Bonus: add fee(int exitHour) that returns 20 for every started hour.
public class Ticket {
    // your code here
}

Main.java

public class Main {
    public static void main(String[] args) {
        Vehicle car = new Vehicle("ABC-1234", "CAR");
        ParkingSpot spot1 = new ParkingSpot(1);
        spot1.park(car);

        // When Ticket is done, uncomment these lines:
        // Ticket t = new Ticket(1, car, spot1, 9);
        // t.print();
        // System.out.println("Fee at 12:00 = " + t.fee(12));
    }
}

Starter (C#)

Ticket.cs

// Practice (Deep Dive 1): build the Ticket class.
//
// A ticket remembers:
//   - Id          (int)
//   - Plate       (string)  the vehicle's plate
//   - SpotNumber  (int)
//   - EntryHour   (int)     0 to 23
//
// 1. Add the four fields.
// 2. Add a constructor Ticket(int id, Vehicle v, ParkingSpot s, int entryHour)
//    that copies the plate from v and the number from s.
// 3. Add Print(), which prints:  Ticket #1 | ABC-1234 | spot 1 | in at 9:00
// Bonus: add Fee(int exitHour) that returns 20 for every started hour.
public class Ticket
{
    // your code here
}

Program.cs

public class Program
{
    public static void Main(string[] args)
    {
        Vehicle car = new Vehicle("ABC-1234", "CAR");
        ParkingSpot spot1 = new ParkingSpot(1);
        spot1.Park(car);

        // When Ticket is done, uncomment these lines:
        // Ticket t = new Ticket(1, car, spot1, 9);
        // t.Print();
        // Console.WriteLine("Fee at 12:00 = " + t.Fee(12));
    }
}

Solution

Try it yourself first!

Show the Java solution

Ticket.java

public class Ticket {
    int id;
    String plate;
    int spotNumber;
    int entryHour;

    Ticket(int id, Vehicle v, ParkingSpot s, int entryHour) {
        this.id = id;
        this.plate = v.plate;
        this.spotNumber = s.number;
        this.entryHour = entryHour;
    }

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

    // Bonus: 20 for every started hour, at least one hour.
    int fee(int exitHour) {
        return Math.max(1, exitHour - entryHour) * 20;
    }
}

Main.java

public class Main {
    public static void main(String[] args) {
        Vehicle car = new Vehicle("ABC-1234", "CAR");
        ParkingSpot spot1 = new ParkingSpot(1);
        spot1.park(car);

        Ticket t = new Ticket(1, car, spot1, 9);
        t.print();
        System.out.println("Fee at 12:00 = " + t.fee(12));
    }
}

Output:

Spot 1: parked ABC-1234
Ticket #1 | ABC-1234 | spot 1 | in at 9:00
Fee at 12:00 = 60

Show the C# solution

Ticket.cs

public class Ticket
{
    public int Id;
    public string Plate;
    public int SpotNumber;
    public int EntryHour;

    public Ticket(int id, Vehicle v, ParkingSpot s, int entryHour)
    {
        Id = id;
        Plate = v.Plate;
        SpotNumber = s.Number;
        EntryHour = entryHour;
    }

    public void Print()
    {
        Console.WriteLine("Ticket #" + Id + " | " + Plate + " | spot "
            + SpotNumber + " | in at " + EntryHour + ":00");
    }

    // Bonus: 20 for every started hour, at least one hour.
    public int Fee(int exitHour) => Math.Max(1, exitHour - EntryHour) * 20;
}

Program.cs

public class Program
{
    public static void Main(string[] args)
    {
        Vehicle car = new Vehicle("ABC-1234", "CAR");
        ParkingSpot spot1 = new ParkingSpot(1);
        spot1.Park(car);

        Ticket t = new Ticket(1, car, spot1, 9);
        t.Print();
        Console.WriteLine("Fee at 12:00 = " + t.Fee(12));
    }
}

Output:

Spot 1: parked ABC-1234
Ticket #1 | ABC-1234 | spot 1 | in at 9:00
Fee at 12:00 = 60

How to run

Java (17 or newer): in the folder, javac -d out *.java then java -cp out Main
C# (.NET SDK 9 or newer): in the folder, dotnet run

Next: Deep Dive 2: Encapsulation, this & static

Build a Parking Lot, part 1 of 5. Code and practice for the video, in Java and C#.

What you'll learn

Find the classes in a problem: nouns become classes, details become fields, verbs become methods
Write a class with fields and a constructor, and create objects with new
Overload a constructor and chain it with this(...)
Give one object a reference to another (a ParkingSpot holds a Vehicle)

Download

parking-lot-dd01-java.zip: Java (lesson code, practice starter, solution)
parking-lot-dd01-csharp.zip: C# (the same)

The code from the video

Java

Vehicle.java

// Deep Dive 1: a class is a blueprint, an object is a thing built from it.
public class Vehicle {
    // Fields: what every vehicle knows about itself.
    // (Open to everyone for now; Deep Dive 2 locks them down.)
    String plate;
    String type;   // "CAR" or "BIKE"

    // Constructor: same name as the class, no return type.
    // It runs once, at the moment `new Vehicle(...)` creates the object.
    Vehicle(String plate, String type) {
        this.plate = plate;   // this.plate = the field, plate = the parameter
        this.type = type;
    }

    // Overloaded constructor: only a plate, so assume a car.
    Vehicle(String plate) {
        this(plate, "CAR");   // hand the work to the other constructor
    }

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

ParkingSpot.java

public class ParkingSpot {
    int number;
    Vehicle parked;   // null while the spot is free

    ParkingSpot(int number) {
        this.number = number;
    }

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

    void park(Vehicle v) {
        parked = v;
        System.out.println("Spot " + number + ": parked " + v.plate);
    }

    void leave() {
        System.out.println("Spot " + number + ": " + parked.plate + " left");
        parked = null;
    }
}

Main.java

public class Main {
    public static void main(String[] args) {
        // One blueprint, three separate objects.
        Vehicle car  = new Vehicle("ABC-1234", "CAR");
        Vehicle bike = new Vehicle("XYZ-77", "BIKE");
        Vehicle taxi = new Vehicle("TAX-5555");   // type defaults to CAR

        car.describe();
        bike.describe();
        taxi.describe();

        ParkingSpot spot1 = new ParkingSpot(1);
        ParkingSpot spot2 = new ParkingSpot(2);

        spot1.park(car);
        spot2.park(bike);
        System.out.println("Spot 1 free? " + spot1.isFree());

        spot1.leave();
        System.out.println("Spot 1 free? " + spot1.isFree());
    }
}

Output:

CAR ABC-1234
BIKE XYZ-77
CAR TAX-5555
Spot 1: parked ABC-1234
Spot 2: parked XYZ-77
Spot 1 free? false
Spot 1: ABC-1234 left
Spot 1 free? true

C#

Vehicle.cs

// Deep Dive 1: a class is a blueprint, an object is a thing built from it.
public class Vehicle
{
    // Fields: what every vehicle knows about itself.
    // (Open to everyone for now; Deep Dive 2 locks them down.)
    public string Plate;
    public string Type;   // "CAR" or "BIKE"

    // Constructor: same name as the class, no return type.
    // It runs once, at the moment `new Vehicle(...)` creates the object.
    public Vehicle(string plate, string type)
    {
        this.Plate = plate;   // this.Plate = the field, plate = the parameter
        this.Type = type;
    }

    // Overloaded constructor: only a plate, so assume a car.
    public Vehicle(string plate) : this(plate, "CAR")   // hand the work to the other constructor
    {
    }

    public void Describe()
    {
        Console.WriteLine(Type + " " + Plate);
    }
}

ParkingSpot.cs

public class ParkingSpot
{
    public int Number;
    public Vehicle Parked;   // null while the spot is free

    public ParkingSpot(int number)
    {
        this.Number = number;
    }

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

    public void Park(Vehicle v)
    {
        Parked = v;
        Console.WriteLine("Spot " + Number + ": parked " + v.Plate);
    }

    public void Leave()
    {
        Console.WriteLine("Spot " + Number + ": " + Parked.Plate + " left");
        Parked = null;
    }
}

Program.cs

public class Program
{
    public static void Main(string[] args)
    {
        // One blueprint, three separate objects.
        Vehicle car  = new Vehicle("ABC-1234", "CAR");
        Vehicle bike = new Vehicle("XYZ-77", "BIKE");
        Vehicle taxi = new Vehicle("TAX-5555");   // type defaults to CAR

        car.Describe();
        bike.Describe();
        taxi.Describe();

        ParkingSpot spot1 = new ParkingSpot(1);
        ParkingSpot spot2 = new ParkingSpot(2);

        spot1.Park(car);
        spot2.Park(bike);
        Console.WriteLine("Spot 1 free? " + spot1.IsFree());

        spot1.Leave();
        Console.WriteLine("Spot 1 free? " + spot1.IsFree());
    }
}

Output:

CAR ABC-1234
BIKE XYZ-77
CAR TAX-5555
Spot 1: parked ABC-1234
Spot 2: parked XYZ-77
Spot 1 free? False
Spot 1: ABC-1234 left
Spot 1 free? True

Practice: Build a Ticket class

Practice (Deep Dive 1): build the Ticket class.

A ticket remembers:
  - id          (int)
  - plate       (String)  the vehicle's plate
  - spotNumber  (int)
  - entryHour   (int)     0 to 23

1. Add the four fields.
2. Add a constructor Ticket(int id, Vehicle v, ParkingSpot s, int entryHour)
   that copies the plate from v and the number from s.
3. Add print(), which prints:  Ticket #1 | ABC-1234 | spot 1 | in at 9:00
Bonus: add fee(int exitHour) that returns 20 for every started hour.

(Names above are the Java ones; the C# starter below uses the C# names in its comments.)

Start from the lesson code above. Only these files change; the rest are the same as the lesson.

Starter (Java)

Ticket.java

// Practice (Deep Dive 1): build the Ticket class.
//
// A ticket remembers:
//   - id          (int)
//   - plate       (String)  the vehicle's plate
//   - spotNumber  (int)
//   - entryHour   (int)     0 to 23
//
// 1. Add the four fields.
// 2. Add a constructor Ticket(int id, Vehicle v, ParkingSpot s, int entryHour)
//    that copies the plate from v and the number from s.
// 3. Add print(), which prints:  Ticket #1 | ABC-1234 | spot 1 | in at 9:00
// Bonus: add fee(int exitHour) that returns 20 for every started hour.
public class Ticket {
    // your code here
}

Main.java

public class Main {
    public static void main(String[] args) {
        Vehicle car = new Vehicle("ABC-1234", "CAR");
        ParkingSpot spot1 = new ParkingSpot(1);
        spot1.park(car);

        // When Ticket is done, uncomment these lines:
        // Ticket t = new Ticket(1, car, spot1, 9);
        // t.print();
        // System.out.println("Fee at 12:00 = " + t.fee(12));
    }
}

Starter (C#)

Ticket.cs

// Practice (Deep Dive 1): build the Ticket class.
//
// A ticket remembers:
//   - Id          (int)
//   - Plate       (string)  the vehicle's plate
//   - SpotNumber  (int)
//   - EntryHour   (int)     0 to 23
//
// 1. Add the four fields.
// 2. Add a constructor Ticket(int id, Vehicle v, ParkingSpot s, int entryHour)
//    that copies the plate from v and the number from s.
// 3. Add Print(), which prints:  Ticket #1 | ABC-1234 | spot 1 | in at 9:00
// Bonus: add Fee(int exitHour) that returns 20 for every started hour.
public class Ticket
{
    // your code here
}

Program.cs

public class Program
{
    public static void Main(string[] args)
    {
        Vehicle car = new Vehicle("ABC-1234", "CAR");
        ParkingSpot spot1 = new ParkingSpot(1);
        spot1.Park(car);

        // When Ticket is done, uncomment these lines:
        // Ticket t = new Ticket(1, car, spot1, 9);
        // t.Print();
        // Console.WriteLine("Fee at 12:00 = " + t.Fee(12));
    }
}

Solution

Try it yourself first!

Show the Java solution

Ticket.java

public class Ticket {
    int id;
    String plate;
    int spotNumber;
    int entryHour;

    Ticket(int id, Vehicle v, ParkingSpot s, int entryHour) {
        this.id = id;
        this.plate = v.plate;
        this.spotNumber = s.number;
        this.entryHour = entryHour;
    }

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

    // Bonus: 20 for every started hour, at least one hour.
    int fee(int exitHour) {
        return Math.max(1, exitHour - entryHour) * 20;
    }
}

Main.java

public class Main {
    public static void main(String[] args) {
        Vehicle car = new Vehicle("ABC-1234", "CAR");
        ParkingSpot spot1 = new ParkingSpot(1);
        spot1.park(car);

        Ticket t = new Ticket(1, car, spot1, 9);
        t.print();
        System.out.println("Fee at 12:00 = " + t.fee(12));
    }
}

Output:

Spot 1: parked ABC-1234
Ticket #1 | ABC-1234 | spot 1 | in at 9:00
Fee at 12:00 = 60

Show the C# solution

Ticket.cs

public class Ticket
{
    public int Id;
    public string Plate;
    public int SpotNumber;
    public int EntryHour;

    public Ticket(int id, Vehicle v, ParkingSpot s, int entryHour)
    {
        Id = id;
        Plate = v.Plate;
        SpotNumber = s.Number;
        EntryHour = entryHour;
    }

    public void Print()
    {
        Console.WriteLine("Ticket #" + Id + " | " + Plate + " | spot "
            + SpotNumber + " | in at " + EntryHour + ":00");
    }

    // Bonus: 20 for every started hour, at least one hour.
    public int Fee(int exitHour) => Math.Max(1, exitHour - EntryHour) * 20;
}

Program.cs

public class Program
{
    public static void Main(string[] args)
    {
        Vehicle car = new Vehicle("ABC-1234", "CAR");
        ParkingSpot spot1 = new ParkingSpot(1);
        spot1.Park(car);

        Ticket t = new Ticket(1, car, spot1, 9);
        t.Print();
        Console.WriteLine("Fee at 12:00 = " + t.Fee(12));
    }
}

Output:

Spot 1: parked ABC-1234
Ticket #1 | ABC-1234 | spot 1 | in at 9:00
Fee at 12:00 = 60

How to run

Java (17 or newer): in the folder, javac -d out *.java then java -cp out Main
C# (.NET SDK 9 or newer): in the folder, dotnet run

Next: Deep Dive 2: Encapsulation, this & static

OOPS! Deep Dive 1: Classes, Objects & Constructors

What you'll learn

Download

The code from the video

Java

C#

Practice: Build a Ticket class

Starter (Java)

Starter (C#)

Solution

How to run

Comments

OOPS! Deep Dive 1: Classes, Objects & Constructors

What you'll learn

Download

The code from the video

Java

C#

Practice: Build a Ticket class

Starter (Java)

Starter (C#)

Solution

How to run

Comments