Simple class which models a booking application with booking and cancellation flow

Booking.kt

enum class VehicleType() {
    CAR,
    CAR_XL
}

class BookingNotFound(message: String) : Exception(message)


class Booking {
    companion object {
        private val bookingDB = hashMapOf<VehicleType, ArrayList<Int>>()
    }

    init {
        bookingDB[VehicleType.CAR] = arrayListOf()
        bookingDB[VehicleType.CAR_XL] = arrayListOf()
    }

    fun makeBooking(type: VehicleType): Int {
        val orderId = getRandomInt()
        addBookingToDb(type, orderId)

        return orderId
    }

    private fun getRandomInt() = ThreadLocalRandom.current().nextInt(1, 1000)
    private fun addBookingToDb(type: VehicleType, orderId: Int) {
        bookingDB[type]?.add(orderId)
    }

    fun cancelBooking(type: VehicleType, orderId: Int) {
        val doesOrderExist = doesBookingExists(type, orderId)
        cancelBookingIfFound(doesOrderExist, orderId, type)
    }


    fun doesBookingExists(type: VehicleType, orderId: Int): Boolean? {
        return bookingDB[type]?.contains(orderId)
    }

    private fun cancelBookingIfFound(
        doesOrderExist: Boolean?,
        orderId: Int,
        type: VehicleType
    ) {
        if (doesOrderExist == null) {
            throw BookingNotFound("Order $orderId not found in db. Cannot cancel.")
        } else {
            bookingDB[type]?.remove(orderId)
            println("Order <$orderId> successfully removed")
        }
    }
}