class BankAccount {
    private var balance: Double = 0

    fileprivate var accountNumber: String

    init(accountNumber: String) {
        self.accountNumber = accountNumber
    }

    func deposit(_ amount: Double) {
        balance += amount
    }

    func currentBalance() -> Double {
        return balance
    }
}

let account = BankAccount(accountNumber: "AC-1001")
account.deposit(250.0)
print(account.currentBalance()) // 250.0
// account.balance would fail to compile — balance is private