Need some in Hibernate

Hello there,

I am new in Hibernate. I look at some examples in Hibernate 3.2
There is ALWAYS a Long id in a table class, like

public class State {
     private Long id; // no such column in my STATE table in database
     private String state_code; // two-character state code
     private String state_name;

     then six pairs of getXXX/setXXX();
}


My question is my existing table in database has NO id column.
I am NOT allow to altering the existed table.

How do I do the mapping (How do I re-write my XML mapping file)?
Thank Q very much in advance!

I try it as below XMl file, but I catch the error when I

try {
  sessionFactory = new Configuration().configure().buildSessionFactory();
} catch (Throwable ex) {
      System.err.println("Initial SessionFactory creation failed." + ex);
             throw new ExceptionInInitializerError(ex);
}

Initial SessionFactory creation 
failed.org.hibernate.PropertyNotFoundException: Could not find a getter 
for state_code in class tables.State

-------------------------------------------------
State.hbm.xml

<hibernate-mapping>
     <class name="tables.State" table="STATE">
         <id name="state_code" type="string" column="STATE_CODE">
             <generator class="native" />
         </id>
         <property name="state" type="string" column="STATE" />
     </class>
</hibernate-mapping>