How to perform subtraction between Text Field and Variable - jasper-reports

Question

I am very new with ireport-5.6.0.
I have one text field(Sanctioned_intake) and one i declare variable(ROPORT_COUNT).Sanctioned_intake is nothing but total number of Student per Department and REPORT_COUNT is return total no of rows which is enter into the report
For Example:
Sanctioned Intake:140 (Total Student per Department)
Actual Admitted: 10 (Actual Admission Taking)
So Difference Should Be:
Vacancy : 130(Remaining Vacancy )
now my question is how to make difference between this two
I declare One Variable
and set the following properties
variable class=java.math.BigDecimal
Calculation Sum
Reset Type Report
Variable Expression: $F{sanctioned_intake}.substract($V{REPORT_COUNT})
But i got this error
Error filling print... Error evaluating expression :      Source text : $F{sanctioned_intake}.substract$V{REPORT_COUNT}
net.sf.jasperreports.engine.fill.JRExpressionEvalException: Error evaluating expression :      
Source text : $F{sanctioned_intake}.substract$V{REPORT_COUNT} 
    at net.sf.jasperreports.engine.fill.JREvaluator.evaluateEstimated(JREvaluator.java:327) 
Caused by: groovy.lang.MissingMethodException: 
No signature of method: java.lang.String.substract() is applicable for argument types: (java.lang.Integer) values: [1]
Possible solutions: substring(int), substring(int, int) 

Answer 1

Data Type of $V{REPORT_COUNT} is integer.
Check data types of each field and variable.
Or if datatype of $F{sanctioned_intake} == BigDecimal then change variable expression to below
$F{sanctioned_intake}.substract(new BigDecimal($V{REPORT_COUNT}))

Answer 2

The error is related to that $F{sanctioned_intake} is declared as java.lang.String.
<field name="sanctioned_intake" class="java.lang.String"/>
However I do not think you will achieve your excepted result changing it to java.math.BigDecimal since jasper report has something called EvalutationTime, the $V{REPORT_COUNT} will start with 0 and reach the total report count only when all records have been displayed.
To the display the difference of 2 fields, declare them as Integer (if you can not, you need to parse them before calculating difference) and then use a text field.
example consider that $F's are java.lang.Integer
<textField>
<reportElement x="163" y="16" width="100" height="20" uuid="4196bd23-306b-44f6-8e3e-4d637facacf6"/>
<textFieldExpression><![CDATA[$F{sanctioned_intake}.intValue()-$F{actual_admitted.intValue()}]]></textFieldExpression>
</textField>
If one needs to be calculated use a variable, calculating (count) and resetting it as needs and then set correct evaluation time on your textField (so that the variables have time to be calculated)
example
<textField evaluationTime="Report">
<reportElement x="163" y="16" width="100" height="20" uuid="4196bd23-306b-44f6-8e3e-4d637facacf6"/>
<textFieldExpression><![CDATA[$F{sanctioned_intake}.intValue()-$V{actual_admitted}.intValue()]]></textFieldExpression>
</textField>
Note: you are using language="groovy" maybe you like to use language="java" on your jasperReport tag.