Encapsulation

public class BankAccount {
    // ========== PRIVATE FIELDS (Hidden) ==========
    private String accountNumber;
    private String holderName;
    private double balance;
    private String pin;

    // ========== CONSTRUCTOR ==========
    public BankAccount(String accountNumber, String holderName, String pin) {
        this.accountNumber = accountNumber;
        this.holderName = holderName;
        this.balance = 0.0;
        setPin(pin); // Use setter for validation
    }

    // ========== GETTERS (Controlled Read Access) ==========
    
    public String getAccountNumber() {
        // Return masked version for security
        return "****" + accountNumber.substring(accountNumber.length() - 4);
    }

    public String getHolderName() {
        return holderName;
    }

    public double getBalance() {
        return balance; // Read-only access
    }

    // No getPin() - password should never be exposed!

    // ========== SETTERS (Controlled Write Access) ==========
    
    public void setHolderName(String name) {
        if (name == null || name.trim().isEmpty()) {
            throw new IllegalArgumentException("Name cannot be empty");
        }
        this.holderName = name.trim();
    }

    private void setPin(String pin) {
        if (pin == null || pin.length() != 4 || !pin.matches("\\d+")) {
            throw new IllegalArgumentException("PIN must be 4 digits");
        }
        this.pin = pin;
    }

    // No setBalance() - balance can only change via deposit/withdraw!

    // ========== BUSINESS METHODS (Controlled Operations) ==========
    
    public boolean deposit(double amount) {
        if (amount <= 0) {
            System.out.println("Deposit amount must be positive");
            return false;
        }
        this.balance += amount;
        System.out.println("Deposited: " + amount + ", New Balance: " + balance);
        return true;
    }

    public boolean withdraw(double amount, String enteredPin) {
        // Validate PIN first
        if (!this.pin.equals(enteredPin)) {
            System.out.println("Invalid PIN!");
            return false;
        }
        
        // Validate amount
        if (amount <= 0) {
            System.out.println("Amount must be positive");
            return false;
        }
        
        if (amount > balance) {
            System.out.println("Insufficient funds!");
            return false;
        }
        
        this.balance -= amount;
        System.out.println("Withdrawn: " + amount + ", Remaining: " + balance);
        return true;
    }
}

// ========== USAGE ==========
BankAccount account = new BankAccount("1234567890", "Alice", "1234");

// ✅ Valid operations through public interface
account.deposit(1000);
account.withdraw(200, "1234");
System.out.println("Balance: " + account.getBalance());

// ❌ These would NOT compile - fields are private!
// account.balance = 1000000;  // Can't access!
// account.pin = "0000";        // Can't access!
// String pin = account.pin;    // Can't access!