I am facing the problem with check box it's giving me null
<rich:dataTable id="itDamageDetailTable"
                                   var="_damageDetail" selectionMode="single" width="950px" selectedClass="selection"
                                    value="#{itShipDamageDetails}" rendered="#{itShipDamageDetails.rowCount > 0}"
                                   headerClass="removeheader">
                                   
                    <rich:column id="check">
                         <h:selectBooleanCheckbox id="Checkbox" value="#{shipmentDamage.detailSelection[_damageDetail]}"/>
                </rich:column>
                    
                    <rich:column id="itShipmentDetail" width="120px" headerClass="head"
                                    footerClass="removefooter" label="#{messages['com.infyz.toms.properties.Shipment_number']}">
                         <f:facet name="header">
                            <h:outputText styleClass="headerText" value="#{messages['com.infyz.toms.properties.Shipment_number']}" />
                         </f:facet>
                         <h:outputText value="#{_damageDetail.itShipmentDetail.itShipmentOrder.shipmentNumber}" />
                    </rich:column>


and my seesion is
@Stateful
@Name("shipmentDamage") 
@Scope(ScopeType.SESSION)
public class ShipmentDamageAction implements ShipmentDamageInterface {

private List<ItDamageDetails> damageDetailsList=new ArrayList<ItDamageDetails>(0);
     
           @DataModel("itShipDamageDetails")
          private List<ItDamageDetails> itShipDamageDetails;

          @Factory("itShipDamageDetails")
         @Observer("itShipDamageDetailConfirmed")
          public void getItShipDamageDetails() {
               try{
                    if(shipDetail !=null){
                    itShipDamageDetails= em.createQuery("from ItDamageDetails as VP  where VP.itShipmentDetail.shipmentDetailId=:damageDetails")
                 .setParameter("damageDetails", shipDetail.getShipmentDetailId()).getResultList();
                    itShipDamageDetails.addAll(damageDetailsList);
                    }
               }
               catch (java.lang.NullPointerException e) {}
          }

private Map<ItDamageDetails,Boolean> detailSelection  = new HashMap<ItDamageDetails,Boolean>();
          
          
          //@SuppressWarnings("unchecked")
         public Map<ItDamageDetails,Boolean> getDetailSelection() {
             return detailSelection;
         }
and i am using a4J:commandButton for cancel which call the action to delete selected rows from the table
<a4j:commandButton id="cancelHold" action="#{shipmentDamage.cancel}" reRender="itDamageDetailTable" immediate="true"
                      value="#{messages['com.infyz.toms.label.button.tooltip.Cancel']}"/>


action is

@End
     public String cancel(){
          List<ItDamageDetails> temp = new ArrayList<ItDamageDetails>();
          for (ItDamageDetails line: itShipDamageDetails) {
               Boolean select = detailSelection.get(line);
               log.info(".................................................................................."+select);
               if(select == null || !select)
               {
                    temp.add(line);
               }
        }
          itShipDamageDetails.clear();
          itShipDamageDetails.addAll(temp);
          detailSelection = new HashMap<ItDamageDetails,Boolean>();
          return "/operations/itShipmentDamageDetail.xhtml";
     }

here when i am displaying the 'selected' value it's always displaying the null
please help........... me