Object-Oriented Programming
with Java

public class BankAccount {
    // fields are private — hidden from outside
    private double balance;

    public BankAccount(double openingBalance) {
        this.balance = openingBalance;
    }

    // public methods control access to the data
    public double getBalance() {
        return balance;
    }

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

    public void withdraw(double amount) {
        if (amount > 0 && amount <= balance) {
            balance -= amount;
        } else {
            System.out.println("Invalid withdrawal");
        }
    }
}

// usage
BankAccount acc = new BankAccount(1000);
acc.deposit(500);
acc.withdraw(200);
System.out.println(acc.getBalance()); // 1300.0
// acc.balance = -50; // ❌ not allowed, balance is private

class Counter {
    static int count = 0;      // shared across all instances
    final String id;          // set once, never changed

    Counter() {
        count++;
        id = "COUNTER-" + count;
    }
}

Counter a = new Counter();
Counter b = new Counter();
System.out.println(Counter.count); // 2 — shared field
System.out.println(a.id);          // COUNTER-1
// a.id = "X"; // ❌ compile error — final cannot be reassigned

class Point {
    int x, y;
    Point(int x, int y) { this.x = x; this.y = y; }

    @Override
    public String toString() {
        return "Point(" + x + ", " + y + ")";
    }

    @Override
    public boolean equals(Object o) {
        if (!(o instanceof Point)) return false;
        Point p = (Point) o;
        return x == p.x && y == p.y;
    }

    @Override
    public int hashCode() {
        return Objects.hash(x, y);
    }
}

Point p1 = new Point(1, 2);
Point p2 = new Point(1, 2);
System.out.println(p1);            // Point(1, 2)
System.out.println(p1.equals(p2)); // true