How to apply conditional style when group changes? - jasper-reports

Question

There are lots of tutorials on how to zebra stripe the rows of a report. Like this:
+-------+-------+
| Value | Color |
+-------+-------+
| A | white |
| A | black |
| B | white |
| B | black |
| B | white |
| C | black |
| D | white |
| D | black |
+-------+-------+
But what I want to do is a grouped stripping. Like this:
+-------+-------+
| Value | Color |
+-------+-------+
| A | white |
| A | white |
| B | black |
| B | black |
| B | black |
| C | white |
| D | black |
| D | black |
+-------+-------+
I'm using the column "Value" as the expression of a group and my data is sorted by "Value". "black" is a black rectangle to be printed when the group is black. "white" is the absence of the black rectangle. I want a variable that I can put in "Print When Expression" of the black rectangle.
What I've tried until now:
Create a variable $V{print}
Initial Value Expression: false
Variable Expression: !$V{print}
Increment type: Group
Increment group: Value
I expected the value of $V{print} to change to it's opposite value every time the group changed. What I get is the normal striped list (black, white, black, white...)

Answer 1

The problem with you current solution is:
calculationType="Nothing"
Nothing: This is the default calculation type that a variable performs. It means that the variable's value is recalculated with every iteration in the data source and that the value returned is obtained by simply evaluating the variable's expression.
This type of calculation will invalidate your incrementType, hence the incrementType has no effect, since we are not calculating. This is why currently you get black,white,black,white.
This will achieve your desired result
Variable definition (lets do some calculation example increment by 1 every time group change, sum or count)
<variable name="GroupCnt" class="java.lang.Integer" incrementType="Group" incrementGroup="myGroup" calculation="Sum">
<variableExpression><![CDATA[1]]></variableExpression>
</variable>
conditionExpression (we can use modulus operator the variable GroupCnt)
<conditionExpression><![CDATA[$V{GroupCnt}%2==0]]></conditionExpression>
Example of complete jrxml (I have added an rectangle as OP comment on this)
<?xml version="1.0" encoding="UTF-8"?>
<jasperReport xmlns="http://jasperreports.sourceforge.net/jasperreports" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://jasperreports.sourceforge.net/jasperreports http://jasperreports.sourceforge.net/xsd/jasperreport.xsd" name="group" pageWidth="595" pageHeight="842" columnWidth="555" leftMargin="20" rightMargin="20" topMargin="20" bottomMargin="20" uuid="c1d9b4b7-6162-4b17-b871-3cf3b867d1ef">
<property name="ireport.zoom" value="1.0"/>
<property name="ireport.x" value="0"/>
<property name="ireport.y" value="0"/>
<style name="myStyle">
<conditionalStyle>
<conditionExpression><![CDATA[new Boolean($V{GroupCnt}.intValue()%2==0)]]></conditionExpression>
<style mode="Opaque" forecolor="#FFFFFF" backcolor="#000000"/>
</conditionalStyle>
</style>
<field name="Value" class="java.lang.String"/>
<variable name="GroupCnt" class="java.lang.Integer" incrementType="Group" incrementGroup="myGroup" calculation="Sum">
<variableExpression><![CDATA[1]]></variableExpression>
</variable>
<group name="myGroup">
<groupExpression><![CDATA[$F{Value}]]></groupExpression>
</group>
<detail>
<band height="20" splitType="Stretch">
<textField>
<reportElement style="myStyle" x="0" y="0" width="150" height="20" uuid="7ca1ac35-6249-4ba6-ac87-031f8d410d2e"/>
<textElement textAlignment="Center" verticalAlignment="Middle"/>
<textFieldExpression><![CDATA[$F{Value}]]></textFieldExpression>
</textField>
<rectangle>
<reportElement style="myStyle" x="150" y="0" width="150" height="20" uuid="d322e0df-0d39-4370-90e6-58305d449852"/>
</rectangle>
</band>
</detail>
</jasperReport>
new Boolean($V{GroupCnt}.intValue()%2==0), new Boolean and intValue() is just use to be compatibile with old jasper report versions it is not needed in latest versions
Result