How to convert String to Double value in jasper Reports?

How to convert String to Double value in jasper Reports? I am having two fields in .jrxml file like below

<field name="secRate" class="java.lang.String"/> 
<field name="secPrice" class="java.lang.String"/> 

i need to subtract both the fi开发者_运维技巧eld

     $V{Variable} = $F{secRate} - SF{secPrice} 

i tried this way but not working

      (new Double(Double.parseDouble($F{mktVal}))) 

any idea? please help me guys..

new Double(Double.parseDouble($F{secRate}) + Double.parseDouble($F{secPrice}))