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public class BankAccount {

// Private fields — no external code can access these directly
    private final String accountNumber;
    private String ownerName;
    private double balance;

// Constructor validates initial state
    public BankAccount(String accountNumber, String ownerName, double initialBalance) {
        if (accountNumber == null || accountNumber.isBlank())
            throw new IllegalArgumentException("Account number cannot be blank");
        if (initialBalance < 0)
            throw new IllegalArgumentException("Initial balance cannot be negative");
        this.accountNumber = accountNumber;
        this.ownerName     = ownerName;
        this.balance       = initialBalance;
    }

// Getter — read-only access to final field
    public String getAccountNumber() { return accountNumber; }

// Getter and validated setter for ownerName
    public String getOwnerName() { return ownerName; }
    public void setOwnerName(String name) {
        if (name == null || name.isBlank())
            throw new IllegalArgumentException("Owner name cannot be blank");
        this.ownerName = name;
    }

// Getter only — balance is modified only through business methods
    public double getBalance() { return balance; }

// Business methods enforce invariants
    public void deposit(double amount) {
        if (amount <= 0) throw new IllegalArgumentException("Deposit must be positive");
        balance += amount;
    }

public void withdraw(double amount) {
        if (amount <= 0) throw new IllegalArgumentException("Withdrawal must be positive");
        if (amount > balance) throw new IllegalStateException("Insufficient funds");
        balance -= amount;
    }

@Override
    public String toString() {
        return String.format("BankAccount[%s, owner=%s, balance=%.2f]",
            accountNumber, ownerName, balance);
    }

public static void main(String[] args) {
        BankAccount acc = new BankAccount("ACC-001", "Alice", 500.0);
        acc.deposit(200.0);
        acc.withdraw(100.0);
        System.out.println(acc);
        System.out.println("Balance: " + acc.getBalance());
    }
}