Classes & Objects in Java

// This is a class - a blueprint for creating Dog objects
public class Dog {
    // Fields (Properties) - what a dog knows/has
    String name;
    int age;
    String breed;
    // Constructor - how to create a new dog
    public Dog(String dogName, int dogAge, String dogBreed) {
        name = dogName;
        age = dogAge;
        breed = dogBreed;
    }
    // Methods (Behaviors) - what a dog can do
    public void bark() {
        System.out.println(name + " says: Woof! Woof!");
    }
    public void sleep() {
        System.out.println(name + " is sleeping... Zzz");
    }
}

public class CreatingObjects {
    public static void main(String[] args) {
        // Creating objects from the Dog class
        // Each object is a real dog with its own properties!
        // Object 1: Buddy
        Dog myDog = new Dog("Buddy", 3, "Golden Retriever");
        // Object 2: Max
        Dog yourDog = new Dog("Max", 5, "Labrador");
        // Object 3: Luna
        Dog friendDog = new Dog("Luna", 2, "Poodle");
        // Each dog can do things (call methods)
        myDog.bark();      // Buddy says: Woof! Woof!
        yourDog.bark();    // Max says: Woof! Woof!
        friendDog.sleep(); // Luna is sleeping... Zzz
        // Each dog has its own properties
        System.out.println(myDog.name + " is " + myDog.age + " years old");
        System.out.println(yourDog.name + " is a " + yourDog.breed);
    }
}

public class Student {        // ① Class Name
    // ② Fields/Properties (what the student knows/has)
    private String name;
    private int age;
    private double gpa;
    private String studentId;
    // ③ Constructor (how to create a student)
    public Student(String name, int age, String id) {
        this.name = name;      // 'this' means "this object's"
        this.age = age;
        this.gpa = 0.0;        // Default GPA
        this.studentId = id;
    }
    // ④ Methods (what the student can do)
    public void study(String subject) {
        System.out.println(name + " is studying " + subject);
    }
    public void takeExam() {
        System.out.println(name + " is taking an exam");
    }
    public void updateGPA(double newGPA) {
        this.gpa = newGPA;
        System.out.println(name + "'s GPA updated to: " + gpa);
    }
    // Method to get information
    public String getInfo() {
        return "Student: " + name + ", Age: " + age + ", ID: " + studentId;
    }
}

public class BankAccount {
    // Properties
    private String accountNumber;
    private String ownerName;
    private double balance;
    // Constructor
    public BankAccount(String number, String owner, double initialBalance) {
        this.accountNumber = number;
        this.ownerName = owner;
        this.balance = initialBalance;
    }
    // Behaviors
    public void deposit(double amount) {
        if (amount > 0) {
            balance += amount;
            System.out.println("Deposited $" + amount);
            System.out.println("New balance: $" + balance);
        }
    }
    public void withdraw(double amount) {
        if (amount > 0 && amount <= balance) {
            balance -= amount;
            System.out.println("Withdrew $" + amount);
            System.out.println("New balance: $" + balance);
        } else {
            System.out.println("Insufficient funds!");
        }
    }
    public double getBalance() {
        return balance;
    }
    public void printAccountInfo() {
        System.out.println("Account: " + accountNumber);
        System.out.println("Owner: " + ownerName);
        System.out.println("Balance: $" + balance);
    }
}
// Using the BankAccount class
class BankDemo {
    public static void main(String[] args) {
        // Create two different bank accounts
        BankAccount account1 = new BankAccount("ACC001", "Alice", 1000.0);
        BankAccount account2 = new BankAccount("ACC002", "Bob", 500.0);
        // Alice's transactions
        account1.deposit(200);    // Alice deposits $200
        account1.withdraw(150);   // Alice withdraws $150
        // Bob's transactions
        account2.deposit(300);    // Bob deposits $300
        // Each account has its own balance!
        account1.printAccountInfo();
        account2.printAccountInfo();
    }
}

public class Book {
    private String title;
    private String author;
    private int pages;
    private double price;
    // Constructor 1: All parameters
    public Book(String title, String author, int pages, double price) {
        this.title = title;
        this.author = author;
        this.pages = pages;
        this.price = price;
    }
    // Constructor 2: Without price (defaults to 0)
    public Book(String title, String author, int pages) {
        this.title = title;
        this.author = author;
        this.pages = pages;
        this.price = 0.0;  // Default price
    }
    // Constructor 3: Just title and author
    public Book(String title, String author) {
        this.title = title;
        this.author = author;
        this.pages = 0;     // Default pages
        this.price = 0.0;   // Default price
    }
    public void displayInfo() {
        System.out.println("Title: " + title);
        System.out.println("Author: " + author);
        System.out.println("Pages: " + pages);
        System.out.println("Price: $" + price);
    }
}
class BookDemo {
    public static void main(String[] args) {
        // Using different constructors
        Book book1 = new Book("Java Basics", "John Doe", 300, 29.99);
        Book book2 = new Book("Python Guide", "Jane Smith", 250);
        Book book3 = new Book("C++ Intro", "Bob Johnson");
        book1.displayInfo();
        System.out.println();
        book2.displayInfo();
        System.out.println();
        book3.displayInfo();
    }
}