Passing null values in java method

Hello all,
         How to pass null value to a java method in ftl? Actually I am 
calling a java method in ftl and passed null value for one of the 
parameter. For eg:

<#assign shippingEst = 
Static["org.ofbiz.order.shoppingcart.shipping.ShippingEvents"].getShipGroupEstimate(dispatcher, 
delegator, "SALES_ORDER",
                        shippingMethodTypeId, carrierPartyId, 
carrierRoleTypeId, shippingCmId, productStoreId,
                        *null*,shippableItemInfo,shippableWeight, 
shippableQuantity, shippableTotal, partyId, productStoreShipMethId)>

But in Java :
This method is not treating null as a null value . Here is the schema of 
this method:
 public static Map getShipGroupEstimate(LocalDispatcher dispatcher, 
GenericDelegator delegator, String orderTypeId,
            String shipmentMethodTypeId, String carrierPartyId, String 
carrierRoleTypeId, String shippingContactMechId,
            String productStoreId,* String supplierPartyId*, List 
itemInfo, BigDecimal shippableWeight, BigDecimal shippableQuantity,
            BigDecimal shippableTotal, String partyId, String 
productStoreShipMethId)

Please help.

Thanks in advance
Divesh

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