ক্লাস এবং অবজেক্ট (Classes & Objects)

public class Main {
    // Define a Dog class inside Main for this example
    static class Dog {
        String name;
        String breed;
        int age;

        // Constructor — runs when you create a new Dog
        Dog(String name, String breed, int age) {
            this.name = name;    // 'this' means "this object's" field
            this.breed = breed;
            this.age = age;
        }

        // Method — something a Dog can do
        void bark() {
            System.out.println(name + " says: Woof!");
        }

        void describe() {
            System.out.println(name + " is a " + age + "-year-old " + breed + ".");
        }
    }

    public static void main(String[] args) {
        // Create objects from the blueprint
        Dog rex = new Dog("Rex", "German Shepherd", 5);
        Dog luna = new Dog("Laboni", "Golden Retriever", 3);

        rex.describe();
        rex.bark();

        luna.describe();
        luna.bark();
    }
}

Rex is a 5-year-old German Shepherd.
Rex says: Woof!
Laboni is a 3-year-old Golden Retriever.
Laboni says: Woof!

public class Main {
    static class BankAccount {
        private String owner;
        private double balance;

        BankAccount(String owner, double initialBalance) {
            this.owner = owner;
            this.balance = initialBalance;
        }

        // Getter — read the balance
        public double getBalance() {
            return balance;
        }

        // Getter — read the owner
        public String getOwner() {
            return owner;
        }

        // Controlled deposit — no negative deposits!
        public void deposit(double amount) {
            if (amount > 0) {
                balance += amount;
                System.out.println("Deposited $" + amount);
            } else {
                System.out.println("Invalid amount!");
            }
        }

        // Controlled withdrawal
        public void withdraw(double amount) {
            if (amount > 0 && amount <= balance) {
                balance -= amount;
                System.out.println("Withdrew $" + amount);
            } else {
                System.out.println("Can't withdraw $" + amount);
            }
        }
    }

    public static void main(String[] args) {
        BankAccount acc = new BankAccount("Anika", 100.0);
        System.out.println(acc.getOwner() + "'s balance: $" + acc.getBalance());

        acc.deposit(50);
        acc.withdraw(30);
        acc.withdraw(200);  // too much!
        acc.deposit(-10);   // sneaky!

        System.out.println("Final balance: $" + acc.getBalance());
    }
}

Anika's balance: $100.0
Deposited $50.0
Withdrew $30.0
Can't withdraw $200.0
Invalid amount!
Final balance: $120.0

public class Main {
    static class Player {
        static int totalPlayers = 0;  // shared across ALL players

        String name;     // unique to each player
        int score;       // unique to each player

        Player(String name) {
            this.name = name;
            this.score = 0;
            totalPlayers++;  // every new player increases the count
        }

        void addScore(int points) {
            this.score += points;
        }

        // Static method — belongs to the class, not any one player
        static int getTotalPlayers() {
            return totalPlayers;
        }
    }

    public static void main(String[] args) {
        System.out.println("Players: " + Player.getTotalPlayers());

        Player p1 = new Player("Mario");
        Player p2 = new Player("Luigi");
        Player p3 = new Player("Peach");

        p1.addScore(100);
        p2.addScore(80);

        System.out.println("Players: " + Player.getTotalPlayers());
        System.out.println(p1.name + ": " + p1.score);
        System.out.println(p2.name + ": " + p2.score);
    }
}

Players: 0
Players: 3
Mario: 100
Luigi: 80