Java Access Modifiers: public, private, protected & Default Explained

class BankAccount {
    // PRIVATE: Internal state, fully protected
    private double balance;
    private String pin;
    private int failedAttempts;

    // Package-private (default): Only used within same package
    static final int MAX_FAILED_ATTEMPTS = 3;

    // PUBLIC: The official deposit API (User requested snippet)
    public void deposit(double amount) {
        if (amount > 0) {
            balance += amount;
            logTransaction("DEPOSIT", amount);
        } else {
            System.out.println("  [WARN] Deposit amount must be positive!");
        }
    }

    // PUBLIC: Getter for balance (Controlled read-only access)
    public double getBalance() {
        return balance;
    }

    // PRIVATE: Internal implementation detail, never exposed
    private void logTransaction(String type, double amount) {
        System.out.printf("  [INTERNAL LOG] %s: $%.2f | New Balance: $%.2f%n",
                type, amount, balance);
    }

    // PUBLIC: Constructor
    public BankAccount(double initialDeposit, String pin) {
        if (initialDeposit >= 0) this.balance = initialDeposit;
        this.pin = pin;
        this.failedAttempts = 0;
    }

    // PUBLIC: PIN verification
    public boolean verifyPin(String inputPin) {
        if (failedAttempts >= MAX_FAILED_ATTEMPTS) {
            System.out.println("  [SECURITY] Account locked. Too many failed PIN attempts!");
            return false;
        }
        if (pin.equals(inputPin)) {
            failedAttempts = 0;
            return true;
        }
        failedAttempts++;
        System.out.printf("  [WARN] Wrong PIN. Attempts remaining: %d%n",
                MAX_FAILED_ATTEMPTS - failedAttempts);
        return false;
    }

    // PUBLIC: toString for display
    @Override
    public String toString() {
        return String.format("BankAccount{balance=$%.2f}", balance);
    }
}

public class Main {
    public static void main(String[] args) {
        System.out.println("=== User Requested BankAccount Demo ===");
        BankAccount account = new BankAccount(1000.0, "9876");

        // Public API works perfectly
        account.deposit(500.0);
        account.deposit(250.0);
        System.out.println("Current Balance: $" + account.getBalance());

        System.out.println("
=== PIN Security System ===");
        account.verifyPin("1234");  // Wrong PIN
        account.verifyPin("4321");  // Wrong PIN
        account.verifyPin("9876");  // Correct PIN

        System.out.println("
=== Private Field Protection Demonstration ===");
        // These would cause COMPILE ERRORS if uncommented:
        // account.balance = 999999;    // private field
        // account.logTransaction(...); // private method
        System.out.println("Private fields cannot be accessed externally!");
        System.out.println("Final account state: " + account);
    }
}

// Package-private class (library-internal utility)
class PasswordHasher {
    // Only used within same package — no need to expose publicly
    static String hash(String password) {
        // Simplified hash simulation (not production!)
        int hash = password.hashCode();
        return "HASH_" + Math.abs(hash);
    }
}

// Public class exposing a clean API
public class PracticalApplication {
    public static void main(String[] args) {
        System.out.println("=== Access Modifier Demo ===");
        String raw = "mySecretPass123";
        String hashed = PasswordHasher.hash(raw); // Accessible (same package)
        System.out.println("Raw     : " + raw);
        System.out.println("Hashed  : " + hashed);
    }
}