 Calculate the age of all visible contacts.
      String ageStr = "";
      List&lt;ContactInfo&gt; items = dataGrid.getVisibleItems();
      if (items.size() &gt; 0) {
        int totalAge = 0;
        for (ContactInfo item : items) {
          totalAge += item.getAge();
        }
        ageStr = "Avg: " + totalAge / items.size();
      }

      // Cells before age column.
      TableRowBuilder tr = startRow();
      tr.startTH().colSpan(4).className(footerStyle).endTH();

      // Show the average age of