Java Classes and Objects

// Class — the blueprint
public class Car {
    String brand;
    String color;
    int speed;

    void accelerate() {
        speed += 10;
        System.out.println(brand + " speed: " + speed + " km/h");
    }
}

// Object — an instance
Car myCar = new Car();
myCar.brand = "Toyota";
myCar.color = "Red";
myCar.accelerate();  // Toyota speed: 10 km/h

public class Student {
    String name;
    int age;

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

    // Default constructor
    public Student() {
        this.name = "Unknown";
        this.age = 0;
    }

    void display() {
        System.out.println(name + ", Age: " + age);
    }
}

// Usage
Student s1 = new Student("Arjun", 22);
Student s2 = new Student();  // Uses default constructor
s1.display();  // Arjun, Age: 22
s2.display();  // Unknown, Age: 0

public class Employee {
    String name;
    double salary;

    public Employee(String name, double salary) {
        this.name = name;       // this.name = instance variable
        this.salary = salary;   // name = parameter
    }
}

public class BankAccount {
    private double balance;

    public double getBalance() {
        return balance;
    }

    public void deposit(double amount) {
        if (amount > 0) {
            balance += amount;
        }
    }
}

public class Counter {
    static int count = 0;

    public Counter() {
        count++;
    }

    public static int getCount() {
        return count;
    }
}

new Counter();
new Counter();
new Counter();
System.out.println(Counter.getCount());  // 3

public class Product {
    private String name;
    private double price;
    private int quantity;
    private static int totalProducts = 0;

    public Product(String name, double price, int quantity) {
        this.name = name;
        this.price = price;
        this.quantity = quantity;
        totalProducts++;
    }

    public double getTotalValue() {
        return price * quantity;
    }

    public void display() {
        System.out.printf("%-15s ₹%8.2f x %d = ₹%.2f%n",
            name, price, quantity, getTotalValue());
    }

    public static void main(String[] args) {
        Product[] products = {
            new Product("Laptop", 65000, 2),
            new Product("Mouse", 500, 10),
            new Product("Keyboard", 1500, 5),
        };

        System.out.println("=== Inventory ===");
        double grandTotal = 0;
        for (Product p : products) {
            p.display();
            grandTotal += p.getTotalValue();
        }
        System.out.printf("%nTotal Products: %d%n", totalProducts);
        System.out.printf("Grand Total: ₹%.2f%n", grandTotal);
    }
}