The correct expression is:
new java.text.DecimalFormat("$ #,##0.00").format(Double.valueOf($P{strParam}))
The working sample for java.lang.Integer and java.lang.String:
<?xml version="1.0" encoding="UTF-8"?>
<jasperReport xmlns="http://jasperreports.sourceforge.net/jasperreports" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://jasperreports.sourceforge.net/jasperreports http://jasperreports.sourceforge.net/xsd/jasperreport.xsd" name="format_as_current" pageWidth="595" pageHeight="842" whenNoDataType="AllSectionsNoDetail" columnWidth="555" leftMargin="20" rightMargin="20" topMargin="20" bottomMargin="20">
<parameter name="intParam" class="java.lang.Integer">
<defaultValueExpression><![CDATA[12345678]]></defaultValueExpression>
</parameter>
<parameter name="strParam" class="java.lang.String">
<defaultValueExpression><![CDATA["12345678.95"]]></defaultValueExpression>
</parameter>
<title>
<band height="79" splitType="Stretch">
<textField>
<reportElement x="137" y="18" width="291" height="20"/>
<textElement/>
<textFieldExpression><![CDATA[new java.text.DecimalFormat("$ #,##0.00").format($P{intParam})]]></textFieldExpression>
</textField>
<textField>
<reportElement x="137" y="48" width="291" height="20"/>
<textElement/>
<textFieldExpression><![CDATA[new java.text.DecimalFormat("$ #,##0.00").format(Double.valueOf($P{strParam} != null && $P{strParam}.length() > 0 ? Double.valueOf($P{strParam}) : 0))]]></textFieldExpression>
</textField>
</band>
</title>
</jasperReport>
The result will be (preview in iReport):
Note:
You should also add check for null.
You can also use pattern property of textField for formatting data.
The sample:
<?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="format_as_current" pageWidth="595" pageHeight="842" whenNoDataType="AllSectionsNoDetail" columnWidth="555" leftMargin="20" rightMargin="20" topMargin="20" bottomMargin="20">
<parameter name="intParam" class="java.lang.Integer">
<defaultValueExpression><![CDATA[12345678]]></defaultValueExpression>
</parameter>
<parameter name="strParam" class="java.lang.String">
<defaultValueExpression><![CDATA["12345678.95"]]></defaultValueExpression>
</parameter>
<title>
<band height="148" splitType="Stretch">
<textField pattern="$ #,##0.00">
<reportElement x="218" y="99" width="100" height="20"/>
<textElement/>
<textFieldExpression><![CDATA[$P{intParam}]]></textFieldExpression>
</textField>
<textField pattern="$ #,##0.00">
<reportElement x="218" y="119" width="100" height="20"/>
<textElement/>
<textFieldExpression><![CDATA[$P{strParam} != null && $P{strParam}.length() > 0 ? Double.valueOf($P{strParam}) : 0]]></textFieldExpression>
</textField>
</band>
</title>
</jasperReport>
The result will be the same.