How to know which row in a table is changed using java and postgresql

I have the following class which listens to notifications from POSTGRESQL database.

class Listener extends Thread {
        private Connection conn;
        private org.postgresql.PGConnection pgconn;

Listener(Connection conn) throws Exception {
    this.setConn(conn);
    this.pgconn = (org.postgresql.PGConnection) conn;
    Statement stmt = conn.createStatement();
    stmt.execute("LISTEN userdata");
    stmt.close();
}

public Connection getConn() {
    return conn;
}

public void setConn(Connection conn) {
    this.conn = conn;
}
public static void main(String[] args) {
        try {
            Class.forName("org.postgresql.Driver");
            String url = "jdbc:postgresql://localhost/users";
            String user = "admin";
            String password = "password";
            Connection listenerCon = DriverManager.getConnection(url, user,
                    password);
            Listener listener = new Listener(listenerCon);
            listener.start();
        } catch (Exception e) {
            e.printStackTrace();
        } 
    }

public void run() {
    while (true) {
        try {
            Statement stmt = conn.createStatement();
            ResultSet rs = stmt
                    .executeQuery("SELECT * FROM userdata WHERE id='1'");
            rs.close();
            stmt.close();
            org.postgresql.PGNotification notifications[] = pgconn
                    .getNotifications();

            if (notifications != null) {
                for (int i = 0; i < notifications.length; i++) {
                    System.out.println("Got notification: "
                            + notifications[i].getParameter());
                }
            }
            Thread.sleep(500);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}
}

These are the following columns in userdata table:

id
cn
sn
givenname
c
l
st
street
o
ou
title
description
postaladdress
telephonenumber
email
userpassword
adorldap

Here is my Stored Procedure in users database

    CCREATE OR REPLACE FUNCTION get_effected_row_primarykey() RETURNS TRIGGER AS
$$
DECLARE
    primarykey TEXT;
BEGIN
    IF (TG_OP='UPDATE') THEN
        IF OLD.cn <> NEW.cn THEN
            SELECT PG_NOTIFY('userdata', 'cn');
        END IF;
        IF OLD.sn <> NEW.sn THEN
            SELECT PG_NOTIFY('userdata', 'sn');
        END IF;
        IF OLD.givenname <> NEW.givenname THEN
            SELECT PG_NOTIFY('userdata', 'givenname');
        END IF;
        IF OLD.c <> NEW.c THEN
            SELECT PG_NOTIFY('userdata', 'c');
        END IF;
        IF OLD.l <> NEW.l THEN
            SELECT PG_NOTIFY('userdata', 'l');
        END IF;
        IF OLD.st <> NEW.st THEN
            SELECT PG_NOTIFY('userdata', 'st');
        END IF;
        IF OLD.street <> NEW.street THEN
            SELECT PG_NOTIFY('userdata', 'street');
        END IF;
        IF OLD.o <> NEW.o THEN
            SELECT PG_NOTIFY('userdata', 'o');
        END IF;
        IF OLD.ou <> NEW.ou THEN
            SELECT PG_NOTIFY('userdata', 'ou');
        END IF;
        IF OLD.title <> NEW.title THEN
            SELECT PG_NOTIFY('userdata', 'title');
        END IF;
        IF OLD.description <> NEW.description THEN
            SELECT PG_NOTIFY('userdata', 'description');
        END IF;
        IF OLD.postaladdress <> NEW.postaladdress THEN
            SELECT PG_NOTIFY('userdata', 'postaladdress');
        END IF;
        IF OLD.telephonenumber <> NEW.telephonenumber THEN
            SELECT PG_NOTIFY('userdata', 'telephonenumber');
        END IF;
        IF OLD.email <> NEW.email THEN
            SELECT PG_NOTIFY('userdata', 'email');
        END IF;
        IF OLD.userpassword <> NEW.userpassword THEN
            SELECT PG_NOTIFY('userdata', 'userpassword');
        END IF;
        IF OLD.adorldap <> NEW.adorldap THEN
            SELECT PG_NOTIFY('userdata', 'adorldap');
        END IF;
        RETURN NEW;
    ELSIF (TG_OP='DELETE') THEN
        SELECT PG_NOTIFY('userdata', OLD.id::text);
        RETURN OLD;
    ELSIF (TG_OP='INSERT') THEN
        SELECT PG_NOTIFY('userdata', NEW.id::text);
        RETURN NEW;
    END IF;
    RETURN NULL;
END;
$$
LANGUAGE plpgsql;

Here is my trigger

    CREATE TRIGGER create_data AFTER INSERT OR UPDATE OR DELETE 
ON userdata 
FOR EACH ROW EXECUTE PROCEDURE get_effected_row_primarykey();

Java program above works fine, it gets notified whenever there is an insert, update, or a delete in table. My questions are:

Updated my stored procedure. When I insert into the table, stored procedure is returning the following error:

ERROR: query has no destination for result data
HINT: If you want to discard the results of a SELECT, use PERFORM instead.
CONTEXT: PL/pgSQL function get_effected_row_primarykey() line 59 at SQL statement

Can anyone help resolve it.Any help appreciated