Getting cumulative sum for rows (groups) - jasper-reports

Question

Imagine the following data set:
What's the proper way to get the values of TOTAL_CUMULATIVE in JasperReports?
i.e: in "VW" row it should sum all values (COST - EXPENSE) until "VW", which means: (500 - 150) + (400-200) = 550.
column xml code:
<textField isStretchWithOverflow="true" pattern="#,##0.00" isBlankWhenNull="true">
<reportElement x="746" y="0" width="65" height="13"/>
<box topPadding="1" leftPadding="1" bottomPadding="1" rightPadding="1"/>
<textElement textAlignment="Right" verticalAlignment="Middle"/>
<textFieldExpression><![CDATA[$V{BALANCE_BY_ACCOUNTING}]]></textFieldExpression>
</textField>
The variable:
<variable name="BALANCE_BY_ACCOUNTING" class="java.lang.Double" resetType="Group" resetGroup="groupByAccounting" calculation="Sum">
<variableExpression><![CDATA[$F{cost} - $F{expense}]]></variableExpression>
</variable>
In the "VW" row, the code sets TOTAL_CUMULATIVE column value to: 200. It should not. It should set to the cumulative sum 550. The same applies to the other rows. In "MERCEDES" row it sets value 600 (should be 11500) and so on.

Answer 1

For getting the cumulative sum the simple sum without resering value for every new enrty in Group can be used.
Example
The Group by car field will help us to solve the task and two types of variables:
First one for calculating sums for Group (resetType="Group" resetGroup="carGroup")
Second one for calculating cumulative sum (resetType=Report)
All data will be placed at groupFooter band.
For getting valid results the data should be ordered by car.
Datasource
The using of simple csv datasource is enough for this example.
car,cost,expense
BMW,200,50
BMW,100,50
BMW,200,50
VW,200,100
VW,200,100
MERCEDES,200,50
MERCEDES,500,50
The name of data adapter for this datasource in the example below is cars.csv. The first line from the file is skipped - it is contains the column's name.
Report template
The variable we need to show cumulative sum is:
<variable name="total" class="java.lang.Integer" calculation="Sum">
<variableExpression><![CDATA[$F{cost} - $F{expense}]]></variableExpression>
</variable>
The textField with $V{total} expression is placed in groupFooter.
The jrxml file:
<?xml version="1.0" encoding="UTF-8"?>
<jasperReport xmlns="http://jasperreports.sourceforge.net/jasperreports" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://jasperreports.sourceforge.net/jasperreports http://jasperreports.sourceforge.net/xsd/jasperreport.xsd" name="Grouping, sum" pageWidth="595" pageHeight="842" columnWidth="555" leftMargin="20" rightMargin="20" topMargin="20" bottomMargin="20" >
<property name="com.jaspersoft.studio.data.defaultdataadapter" value="cars.csv"/>
<field name="car" class="java.lang.String"/>
<field name="cost" class="java.lang.Integer"/>
<field name="expense" class="java.lang.Integer"/>
<variable name="costForCar" class="java.lang.Integer" resetType="Group" resetGroup="carGroup" calculation="Sum">
<variableExpression><![CDATA[$F{cost}]]></variableExpression>
</variable>
<variable name="expenseForCar" class="java.lang.Integer" resetType="Group" resetGroup="carGroup" calculation="Sum">
<variableExpression><![CDATA[$F{expense}]]></variableExpression>
</variable>
<variable name="totalForCar" class="java.lang.Integer" resetType="Group" resetGroup="carGroup" calculation="Sum">
<variableExpression><![CDATA[$F{cost} - $F{expense}]]></variableExpression>
</variable>
<variable name="total" class="java.lang.Integer" resetType="Report" calculation="Sum">
<variableExpression><![CDATA[$F{cost} - $F{expense}]]></variableExpression>
</variable>
<group name="carGroup">
<groupExpression><![CDATA[$F{car}]]></groupExpression>
<groupFooter>
<band height="15">
<textField>
<reportElement x="0" y="0" width="100" height="15" />
<textFieldExpression><![CDATA[$F{car}]]></textFieldExpression>
</textField>
<textField>
<reportElement x="100" y="0" width="110" height="15" />
<textFieldExpression><![CDATA[$V{costForCar}]]></textFieldExpression>
</textField>
<textField>
<reportElement x="210" y="0" width="110" height="15" />
<textFieldExpression><![CDATA[$V{expenseForCar}]]></textFieldExpression>
</textField>
<textField>
<reportElement x="320" y="0" width="110" height="15" />
<textFieldExpression><![CDATA[$V{totalForCar}]]></textFieldExpression>
</textField>
<textField>
<reportElement x="430" y="0" width="110" height="15" />
<textFieldExpression><![CDATA[$V{total}]]></textFieldExpression>
</textField>
</band>
</groupFooter>
</group>
<columnHeader>
<band height="15" splitType="Stretch">
<staticText>
<reportElement x="0" y="0" width="100" height="15" />
<textElement textAlignment="Center"/>
<text><![CDATA[Car]]></text>
</staticText>
<staticText>
<reportElement x="100" y="0" width="110" height="15" />
<textElement textAlignment="Center"/>
<text><![CDATA[Cost]]></text>
</staticText>
<staticText>
<reportElement x="210" y="0" width="110" height="15" />
<textElement textAlignment="Center"/>
<text><![CDATA[Expense]]></text>
</staticText>
<staticText>
<reportElement x="320" y="0" width="110" height="15" />
<textElement textAlignment="Center"/>
<text><![CDATA[Total]]></text>
</staticText>
<staticText>
<reportElement x="430" y="0" width="110" height="15" />
<textElement textAlignment="Center"/>
<text><![CDATA[Total, cumulative]]></text>
</staticText>
</band>
</columnHeader>
</jasperReport>
The same effect can be achieved with help of Detail band and evaluationTime attribute of textField element.
Output result
The result in Jaspersoft Studio (JSS):