making a rule and know when it is violated

Hello,
I want to make a rule in phpPgAdmin(PostgreSQL 7.3.8-RH ).
Then I want to intercept any violation on that rule in java.
Here is the table creation code:
CREATE TABLE "tblUser" (
    "UserID" serial NOT NULL,
    "UserName" character varying(50) NOT NULL,
    "UserPass" character varying(50) NOT NULL,
    "UserRoleID" integer DEFAULT 3 NOT NULL
);
Now it's imperative that the field UserRoleID has maximum one record that
contains the integer 1. Possible other values are 2 and 3. They are allowed
to exist in more than one record.
After that I need to know in java how to know when a violation of this rule
occurs.
Anyone any ideas?
Kind regards,
Nico.