1.3.4. Collection of values

We add a collection of value typed objects to the Person entity. We want to store email addresses, so the type we use is String, and the collection is again a Set:
private Set emailAddresses = new HashSet();

public Set getEmailAddresses() {
    return emailAddresses;
}

public void setEmailAddresses(Set emailAddresses) {
    this.emailAddresses = emailAddresses;
}
The mapping of this Set:
<set name="emailAddresses" table="PERSON_EMAIL_ADDR">
    <key column="PERSON_ID"/>
    <element type="string" column="EMAIL_ADDR"/>
</set>
The difference compared with the earlier mapping is the element part, which tells Hibernate that the collection does not contain references to another entity, but a collection of elements of type String (the lowercase name tells you it's a Hibernate mapping type/converter). Once again, the table attribute of the set element determines the table name for the collection. The key element defines the foreign-key column name in the collection table. The column attribute in the element element defines the column name where the String values will actually be stored.
Have a look at the updated schema:
  _____________        __________________
 |             |      |                  |       _____________
 |   EVENTS    |      |   PERSON_EVENT   |      |             |       ___________________
 |_____________|      |__________________|      |    PERSON   |      |                   |
 |             |      |                  |      |_____________|      | PERSON_EMAIL_ADDR |
 | *EVENT_ID   | <--> | *EVENT_ID        |      |             |      |___________________|
 |  EVENT_DATE |      | *PERSON_ID       | <--> | *PERSON_ID  | <--> |  *PERSON_ID       |
 |  TITLE      |      |__________________|      |  AGE        |      |  *EMAIL_ADDR      |
 |_____________|                                |  FIRSTNAME  |      |___________________|
                                                |  LASTNAME   |
                                                |_____________|

You can see that the primary key of the collection table is in fact a composite key, using both columns. This also implies that there can't be duplicate email addresses per person, which is exactly the semantics we need for a set in Java.
You can now try and add elements to this collection, just like we did before by linking persons and events. It's the same code in Java:
private void addEmailToPerson(Long personId, String emailAddress) {

    Session session = HibernateUtil.getSessionFactory().getCurrentSession();
    session.beginTransaction();

    Person aPerson = (Person) session.load(Person.class, personId);

    // The getEmailAddresses() might trigger a lazy load of the collection
    aPerson.getEmailAddresses().add(emailAddress);

    session.getTransaction().commit();
}
This time we didnt' use a fetch query to initialize the collection. Hence, the call to its getter method will trigger an additional select to initialize it, so we can add an element to it. Monitor the SQL log and try to optimize this with an eager fetch.