formatting with IF condition

[mmu]

Hello,

I want to do this :

when timestamp.equals(a) print "A"
when timestamp.equals(b) print "B"
else print "C"

I created this as an expression :
(($F{timestamp}.equals($F{a}))? "A" : "B")

When I run the report I receive a ClassCast Exception for java.sql.timestamp because I return a string for y query of timestamp. I think I can use a custom expression, but I did not manage to import my own classes in ireport, it always says : import invalid. where do I have to place the jar file for the import ?

regards
mmu