about function overload,a bug?

My English is bad.
I write two functions with plpgsql:

1. user_chgpasswd(character varying, character varying, timestamp
without time zone)
Note:content of the function is just "return 0;"
2.user_chgpasswd(character varying, character varying, character
varying)

Both of them return integer.
I invoke the first one in pgadmin,returns 0;but in my
application(setString(),setString(),setTimestamp()),the result is not 0.
I tried postgresql-8.2-506.jdbc3.jar and
postgresql-8.2-506.jdbc4.jar,both wrong.
I check my code for a long time,no wrong.I delete the second function in
postgresql and run my application again,returns 0.
It seems "timestamp without time zone" is same as "character varying" in
jdbc,so jdbc invoke wrong function.