Java Development Tools

I'm using 11.1.1.6 Jdev.

I have a <af:table> with a column, which is Number type but most of them with NULL value.

I try to create a custom sort on this column: NULL always at the bottom no matter it's ascending or descending. But I could not get it work.

Can anyone show me how to implement onSort the above logic?

Thank you

public void onSort(SortEvent sortEvent) { 
             List currentSortList = sortEvent.getSortCriteria();
             SortCriterion sortCriterion = (SortCriterion)currentSortList.get(0);


                          
             List<SortCriterion> tableSortList = new ArrayList<SortCriterion>();
             SortCriterion sortCriterionReplaced = new SortCriterion(sortCriterion.getProperty(), sortCriterion.isAscending());
             tableSortList.add(sortCriterionReplaced);

             RichTable richTable = (RichTable)sortEvent.getComponent();
             richTable.setSortCriteria(tableSortList);
             
               //refresh table
             AdfFacesContext.getCurrentInstance().addPartialTarget(richTable);

}