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.

Class Anatomy Diagram

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 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++;
    }
}

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 class Book {
    String title;
    String author;

    // Constructor with title only, author has default value
    public Book(String title) {
        this.title = title;
        this.author = "Unknown"; // default author
    }
}

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

Unit 5 Anatomy of a Class(Period 1)