CustomerService.kt

data class Customer(private val firstName: String, 
                    private val lastName: String, 
                    private val age: Int) {
    private val fullName = "$firstName $lastName"
    
    fun isAdult(): Boolean {
        return age >= 18
    }
    
    fun getGreetingName(): String {
        return if (isAdult()) fullName else firstName
    }
}

class FrontEnd {
  fun display(message: String) {
    // code to display the message on the Frontend device
  }
}

class DriversLicenseService {
    private val frontEnd = FrontEnd()
    private val driverLicenseApi = DriverLicenseApi
    
    public fun greetCustomer(customer: Customer){
        val greetingMessage = "Good day, ${customer.getGreetingName()}"
        frontEnd.display(greetingMessage)
    }
    
    public fun getDriversLicense(customer: Customer) {
        if(!customer.isAdult()) {
            throw CustomerNotAdultException(customer)
        }
        // code to get the driver license for the customer from external api (or a database)
        return driverLicenseApi.getDriverseLicenseFor(customer.id)        
    }
}