code: public static long getMaxIdForClass(
              PersistenceBroker brokerForClass, ClassDescriptor cldForOriginalOrExtent, FieldDescriptor original)
              throws PersistenceBrokerException
      {
          FieldDescriptor field = null;
          if (!original.getClassDescriptor().equals(cldForOriginalOrExtent))
          {
              // check if extent match not the same table
              if (!original.getClassDescriptor().getFullTableName().equals(
                      cldForOriginalOrExtent.getFullTableName()))
              {
                  // we have to look for id's in extent class table
                  field = cldForOriginalOrExtent.getFieldDescriptorByName(original.getAttributeName());
              }
          }
          else
          {
              field = original;
          }
          if (field == null)
          {
              // if null skip this call
              return 0;
          }
  
          String column = field.getColumnName();
          long result = 0;
          ResultSet rs = null;
          Statement stmt = null;
          StatementManagerIF sm = brokerForClass.serviceStatementManager();
          String table = cldForOriginalOrExtent.getFullTableName();
          // String column = cld.getFieldDescriptorByName(fieldName).getColumnName();
          String sql = SM_SELECT_MAX + column + SM_FROM + table;
          try
          {
              //lookup max id for the current class
              stmt = sm.getGenericStatement(cldForOriginalOrExtent, Query.NOT_SCROLLABLE);
              rs = stmt.executeQuery(sql);
              rs.next();
              result = rs.getLong(1);
          }
          catch (Exception e)
          {
              log.warn("Cannot lookup max value from table " + table + " for column " + column +
                      ", PB was " + brokerForClass + ", using jdbc-descriptor " +
                      brokerForClass.serviceConnectionManager().getConnectionDescriptor(), e);
          }
          finally
          {
              try
              {
                  sm.closeResources(stmt, rs);
              }
              catch (Exception ignore)
              {
                  // ignore it
             }
          }
          return result;
      }
docs: lookup current maximum value for a single field in
  table the given class descriptor was associated.