User Defined Java Expression invokes a void method

Hi, I have a UDJE which invokes a static void method created by me.

Problem is when I call it from a UDJE: it returns me "Operand stack underrun at offset 8".

Is there any way yo call static void methods in UDJE? what should I put in "Value Type" and "New Field" boxes if I´m not generating anything?

Note: I cannot use UDJC because I´m working in a Mapping step, and as due to a bug of Pentaho, UDJC doesn´t work propperly.

Thanks!