Anatomy of a Class

This notebook will explain the structure of a class in Java, including attributes, methods, and instantiation. It will also include examples and a mini-project that requires fixing broken code.

1. Introduction to Classes

• A class in Java is a blueprint for creating objects. It defines a data structure that includes methods and attributes.
• Example:

public class Car {
    // Attributes
    String model;
    int year;

    // Method
    void drive() {
        System.out.println("Driving the car.");
    }
}

This Car class has attributes model and year, and a method drive() that prints a message.

2. Attributes and Methods

• Attributes (or fields) are variables that belong to an object.
• Methods are functions that belong to a class.
• Example:

public class Person {
    // Attributes
    String name;
    int age;

    // Method
    void greet() {
        System.out.println("Hello, my name is " + name);
    }
}

In this Person class, name and age are attributes, and greet() is a method.

3. Constructor

• Constructors are special methods used to initialize objects.
• They have the same name as the class and do not have a return type.
• Example:

public class Person {
    String name;
    int age;

    // Constructor
    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }

    void greet() {
        System.out.println("Hello, my name is " + name);
    }
}

This Person class includes a constructor to initialize name and age.

4. Class vs. Instance Variables

• Instance variables are attributes specific to each object.
• Class variables (static variables) are shared among all instances of the class.
• Example:

public static class Car {
    // Class variable
    static int numberOfCars = 0;

    // Instance variables
    String model;
    int year;

    // Constructor
    public Car(String model, int year) {
        this.model = model;
        this.year = year;
        numberOfCars++;
    }
}

Car mortCar = new Car("MortCar",1995);
System.out.println(mortCar.model);
System.out.println(mortCar.year);

MortCar
1995

Here, numberOfCars is a class variable that keeps track of how many Car objects have been created.

5. Mini Project: Fix the Code

• Below is a class with broken code. The goal is to fix the class so it properly initializes and uses instance variables.

• Broken code:

• Task: Debug the Book class so it correctly initializes title and author. Consider how the constructor should be modified.

public static class Book {
    String title;
    String author;

    // Fixed Constructor
    public Book(String title, String author) {
        this.title = title;
        this.author = author;
    }
}


Book mortBook = new Book("Why You Shouldn't Use Paper","John Mortenson");
System.out.println(mortBook.title);
System.out.println(mortBook.author);

Why You Shouldn't Use Paper
John Mortenson

• Task: Debug the Book class so it correctly initializes title and author. Consider how the constructor should be modified.