code: @SuppressWarnings("incomplete-switch")
  protected Icon getIcon(JTable table, int column) {
      SortKey sortKey = getSortKey(table, column);
      if (sortKey != null && table.convertColumnIndexToView(sortKey.getColumn()) == column) {
        switch (sortKey.getSortOrder()) {
          case ASCENDING:
            return UIManager.getIcon("Table.ascendingSortIcon");
          case DESCENDING:
            return UIManager.getIcon("Table.descendingSortIcon");
        }
      }
      return null;
    }
docs: Overloaded to return an icon suitable to the primary sorted column, or null if
  the column is not the primary sort key.
  
  @param table the <code>JTable</code>.
  @param column the column index.
  @return the sort icon, or null if the column is unsorted.