public Order() {
        this.orderNumber = orderNumberCounter;
        orderNumberCounter++;
        orderLines = new ArrayList<OrderLine>();
        orderTime = Calendar.getInstance();
    }

    /**
     * Constructs a new Order object with the value of the passed in parameters
     * 
     * @param orderLines - the initial value of each order line that is in this order
     */
    public Order(ArrayList<OrderLine> orderLines) {
        this.orderNumber = orderNumberCounter;
        orderNumberCounter++;
        
        for (OrderLine orderLine : orderLines) {
            this.orderLines.add(orderLine);
        }
        
        this.orderTime = Calendar.getInstance();
    }

    /**
     * Return the food numbers of foods that are in this order
     * 
     * @return a copy of attribute foodNumbers
     */
    public ArrayList<OrderLine> getOrderLines() {
        return (ArrayList<OrderLine>)orderLines.clone();
    }

    /**
     * Add a food to the order along with the correponding quantity
     * 
     * @param food - the food to add 
     * @param quantity - the quantity of the food
     * @return true if the food item is added successfully, false otherwise
     */
    public boolean addItem(Food food, int quantity) {
        if (!orderLines.contains(searchOrderLine(food))) {
            return this.orderLines.add(new OrderLine(food, quantity));
        }
        return false;
    }
    
    /**
     * Add an order line to the order
     * 
     * @param orderLine - the new order line to add in 
     * @return true if the orderLine is added successfully, false otherwise
     */
    public boolean addItem(OrderLine orderLine) {
        if (!orderLines.contains(searchOrderLine(orderLine.getFood()))) {
            return this.orderLines.add(orderLine);
        }
        return false;
    }

     /**
     * Remove a food from the order
     * 
      * @param food - the food to remove
      * @return true if the food item is removed successfully, false otherwise
     */
    public boolean removeItem(Food food) {
        return this.orderLines.remove(searchOrderLine(food));
    }

    /**
     * Search whether a food already existed in the order
     * 
     * @param food - the food to search
     * @return the order line containing the food 
     */
    private OrderLine searchOrderLine(Food food) {
        for (OrderLine orderLine : orderLines) {
            if (orderLine.getFood().getFoodNumber() == food.getFoodNumber()) {
                return orderLine;
            }
        }
        return new OrderLine();
    }