Custom Numbering Callback Class

You can stamp custom number on the invoices by adding a callback class for Invoice Numbering.

To add the custom class

Go to Setup > App Setup > Develop > Apex Classes.
Click New.
Enter the sample callback class code.
Click Save.

This is just a sample callback class. You may change the code per your requirements.

In order to implement the interface, prefix it with Apttus_Billing.CustomClass.<Interface Name>

/**
*  Apttus Billing
*  DefaultCustomNumberingCallback
*  
 *  @2011-2016 Apttus Inc. All rights reserved.
*/
public with sharing class DefaultCustomNumberingCallback
      implements CustomClass.IFacilitateCustomTransactionNumbers {
 
      /**
      * Evaluate the "object type" of the specified numbering configuration settings and determine
      * if the settings are valid for your custom implementation.  For example, the default  Apttus
      * implementation of this interface for the "Invoice" object type expects that only "field 1" has
      * been checked.  This is critical because the user interface for the custom number entries
      * uses the numbering configuration setting object to determine which fields to force the
      * user to enter.  Any field unchecked will be hidden from the user.
      *
      * @param customNumberingConfigSetting The custom number configuration setting to evaluate.
      *
       * @return true if the "custom numbering configuration settings" is valid for its underlying "object type",
      *   otherwise return false.
      */
      public boolean isValidConfiguration(CustomNumberingConfigurationSetting__c numberingConfigSettings) {
 
            if (numberingConfigSettings.ObjectType__c.equals(CustomClass.CUSTOM_NUMBERING_OBJECT_TYPE_INVOICE)) {
 
                  if (true == numberingConfigSettings.FieldOneEnabled__c &&
                        false == numberingConfigSettings.FieldtwoEnabled__c &&
                        false == numberingConfigSettings.FieldThreeEnabled__c &&
                        false == numberingConfigSettings.FieldFourEnabled__c &&
                        false == numberingConfigSettings.FieldFiveEnabled__c) {
 
                        return true;
                  }
            }
 
            return false;
      }
 
      /**
      * Build the list of "lookup key" field lists for the each of the
      * specified Invoices.
      *
      * @param invoices The list of Invoices to evaluate and to find the
      *  corresponding list of lookup key field lists for.
      * @param customNumberingConfigSetting The custom number configuration
      *  setting for Invoices.
      *
      * @return the list of lists.  Each element of the returned list must
      *  contain the "lookup key" field list for the corresponding Invoice.
      *  The number of lists returned must match the number of Invoices passed in
      *  and the ith list must correspond to the ith Invoice.  For example, if
      *  2 Invoices are passed in, then 2 "field lists" should be returned and
      *  the 1st field list must correspond to the 1st Invoice and the 2nd
      *  field list must correspond to the 2nd Invoice.
      */
      public List<List<String>> fetchInvoiceCustomNumberingFieldLists(
            List<Invoice__c> invoices,
            CustomNumberingConfigurationSetting__c customNumberingConfigSetting)
      {
 
            List<List<String>> fieldLists = new List<List<String>>();
            for (Invoice__c inv : invoices) {
 
                  List<String> fieldList = new List<String>();
                  fieldList.add(inv.BillingCountry__c);
                  fieldLists.add(fieldList);
            }
            return fieldLists;
      }
 
}

CODE

To add the name of custom callback class, go to Setup > App Setup > Develop > Custom Settings and click Manage beside Billing Custom Classes. Click Edit for System Properties and enter the name of your custom callback class in Custom Invoice Numbering Callback Class.