protected String createTotalRow(boolean grandTotal) 
{
        StringBuffer buffer = new StringBuffer(1000);
        buffer.append("\n< tr class=\"total\" >"); //$NON-NLS-1$
        List headerCells = tableModel.getHeaderCellList();
        for (Iterator it = headerCells.iterator(); it.hasNext();)
        {
            HeaderCell cell = (HeaderCell) it.next();
            String cssClass = ObjectUtils.toString(cell.getHtmlAttributes().get("class"));
            buffer.append("< td"); //$NON-NLS-1$
            if (StringUtils.isNotEmpty(cssClass))
            {
                buffer.append(" class=\""); //$NON-NLS-1$
                buffer.append(cssClass);
                buffer.append("\""); //$NON-NLS-1$
            }
            buffer.append(" >"); //$NON-NLS-1$
            if (cell.isTotaled())
            {
                String totalPropertyName = cell.getBeanPropertyName();
                Object total = grandTotal ? grandTotals.get(totalPropertyName) : subTotals.get(totalPropertyName);
                DisplaytagColumnDecorator[] decorators = cell.getColumnDecorators();
                for (int j = 0; j <  decorators.length; j++)
                {
                    try
                    {
                        total = decorators[j].decorate(total, this.getPageContext(), tableModel.getMedia());
                    }
                    catch (DecoratorException e)
                    {
                        log.warn(e.getMessage(), e);
                        // ignore, use undecorated value for totals
                    }
                }
                buffer.append(total);
            }
            else if (groupPropertyName != null && groupPropertyName.equals(cell.getBeanPropertyName()))
            {
                buffer.append(grandTotal ? totalLabel : MessageFormat.format(subtotalLabel, new Object[]{previousValues
                    .get(groupPropertyName)}));
            }
            buffer.append("< /td >"); //$NON-NLS-1$
        }
        buffer.append("< /tr >"); //$NON-NLS-1$
        // reset subtotal
        this.subTotals.clear();
        return buffer.toString();
}

 public final String finishRow() 
{
        StringBuffer buffer = new StringBuffer(1000);
        // Grand totals...
        if (getViewIndex() == ((List) getDecoratedObject()).size() - 1)
        {
            if (groupPropertyName != null)
            {
                buffer.append(createTotalRow(false));
            }
            buffer.append(createTotalRow(true));
        }
        return buffer.toString();
}

After every row completes we evaluate to see if we should be drawing a new total line and summing the results from the previous group.

 public  void init(PageContext context,
    Object decorated,
    TableModel tableModel) 
{
        super.init(context, decorated, tableModel);
        // reset
        groupPropertyName = null;
        grandTotals.clear();
        subTotals.clear();
        previousValues.clear();
        for (Iterator it = tableModel.getHeaderCellList().iterator(); it.hasNext();)
        {
            HeaderCell cell = (HeaderCell) it.next();
            if (cell.getGroup() == 1)
            {
                groupPropertyName = cell.getBeanPropertyName();
            }
        }
}

 public  void setSubtotalLabel(String subtotalLabel) 
{
                  
        this.subtotalLabel = subtotalLabel;
}

Setter for subtotalLabel.

 public  void setTotalLabel(String totalLabel) 
{
        this.totalLabel = totalLabel;
}

Setter for totalLabel.

 public String startRow() 
{
        String subtotalRow = null;
        if (groupPropertyName != null)
        {
            Object groupedPropertyValue = evaluate(groupPropertyName);
            Object previousGroupedPropertyValue = previousValues.get(groupPropertyName);
            // subtotals
            if (previousGroupedPropertyValue != null
                && !ObjectUtils.equals(previousGroupedPropertyValue, groupedPropertyValue))
            {
                subtotalRow = createTotalRow(false);
            }
            previousValues.put(groupPropertyName, groupedPropertyValue);
        }
        for (Iterator it = tableModel.getHeaderCellList().iterator(); it.hasNext();)
        {
            HeaderCell cell = (HeaderCell) it.next();
            if (cell.isTotaled())
            {
                String totalPropertyName = cell.getBeanPropertyName();
                Number amount = (Number) evaluate(totalPropertyName);
                Number previousSubTotal = (Number) subTotals.get(totalPropertyName);
                Number previousGrandTotals = (Number) grandTotals.get(totalPropertyName);
                subTotals.put(totalPropertyName, new Double((previousSubTotal != null
                    ? previousSubTotal.doubleValue()
                    : 0)
                    + (amount != null ? amount.doubleValue() : 0)));
                grandTotals.put(totalPropertyName, new Double((previousGrandTotals != null ? previousGrandTotals
                    .doubleValue() : 0)
                    + (amount != null ? amount.doubleValue() : 0)));
            }
        }
        return subtotalRow;
}