Java HotSpot Virtual Machine

Hi All,

I'm new to iReport tool.

I have three field display in my report .third field should hold sum of other two field values.How to set formula for this field in iReport tool.

I gave $F{a}+$F in expression editor ,but still i get only null value while a holds 5 and b holds 6 . third field value shud be 11 .

When i check the expression in exp editor i get this exception y?

Sourced file: inline evaluation of: ``((java.lang.String)field_a.getValue())+((java.lang.String)field_b.getValue());'' : illegal use of null value or 'null' literal
;
Line: 1

Please guide me on this.