public boolean isReadyForShipment(Long orderId) {
        if (orderId == null) {
            throw new IllegalArgumentException("OrderId cannot be null");
        }
        Order order = orderById.get(orderId);
        if (order == null) {
            throw new IllegalArgumentException("No such order Id: " + orderId);
        }
        return order.getStatus() == Order.Status.READY_FOR_SHIPMENT;
    }