2.8.12. Custom collection types

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If you wish to use other collection types than `List`, `Set` or ``Map`, like `Queue` for instance,
you have to use a custom collection type, as illustrated by the following example:

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<div id="collections-custom-collection-mapping-example" class="exampleblock">
<div class="title">Example 170. Custom collection mapping example</div>
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<div class="content">
<pre class="prettyprint highlight">`@Entity(name = "Person")
public static class Person {

    @Id
    private Long id;

    @OneToMany(cascade = CascadeType.ALL)
    @CollectionType( type = "org.hibernate.userguide.collections.type.QueueType")
    private Collection&lt;Phone&gt; phones = new LinkedList&lt;&gt;();

    //Getters and setters are omitted for brevity

}

@Entity(name = "Phone")
public static class Phone implements Comparable&lt;Phone&gt; {

    @Id
    private Long id;

    private String type;

    @NaturalId
    @Column(name = "`number`")
    private String number;

    //Getters and setters are omitted for brevity

}

public class QueueType implements UserCollectionType {

    @Override
    public PersistentCollection instantiate(
            SharedSessionContractImplementor session,
            CollectionPersister persister) throws HibernateException {
        return new PersistentQueue( session );
    }

    @Override
    public PersistentCollection wrap(
            SharedSessionContractImplementor session,
            Object collection) {
        return new PersistentQueue( session, (List) collection );
    }

    @Override
    public Iterator getElementsIterator(Object collection) {
        return ( (Queue) collection ).iterator();
    }

    @Override
    public boolean contains(Object collection, Object entity) {
        return ( (Queue) collection ).contains( entity );
    }

    @Override
    public Object indexOf(Object collection, Object entity) {
        int i = ( (List) collection ).indexOf( entity );
        return ( i &lt; 0 ) ? null : i;
    }

    @Override
    public Object replaceElements(
            Object original,
            Object target,
            CollectionPersister persister,
            Object owner,
            Map copyCache,
            SharedSessionContractImplementor session)
            throws HibernateException {
        Queue result = (Queue) target;
        result.clear();
        result.addAll( (Queue) original );
        return result;
    }

    @Override
    public Object instantiate(int anticipatedSize) {
        return new LinkedList&lt;&gt;();
    }

}

public class PersistentQueue extends PersistentBag implements Queue {

    public PersistentQueue(SharedSessionContractImplementor session) {
        super( session );
    }

    public PersistentQueue(SharedSessionContractImplementor session, List list) {
        super( session, list );
    }

    @Override
    public boolean offer(Object o) {
        return add(o);
    }

    @Override
    public Object remove() {
        return poll();
    }

    @Override
    public Object poll() {
        int size = size();
        if(size &gt; 0) {
            Object first = get(0);
            remove( 0 );
            return first;
        }
        throw new NoSuchElementException();
    }

    @Override
    public Object element() {
        return peek();
    }

    @Override
    public Object peek() {
        return size() &gt; 0 ? get( 0 ) : null;
    }
}`</pre>
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<div class="admonitionblock note">
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<tr>
<td class="icon">

</td>
<td class="content">
<div class="paragraph">

The reason why the `Queue` interface is not used for the entity attribute is because Hibernate only allows the following types:

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<div class="ulist">

• java.util.List
• java.util.Set
• java.util.Map
• java.util.SortedSet

• java.util.SortedMap </div>

  However, the custom collection type can still be customized as long as the base type is one of the aformentioned persistent types.

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  This way, the Phone collection can be used as a java.util.Queue:

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  Example 171. Custom collection example

  `Person person = entityManager.find( Person.class, 1L );
  Queue<Phone> phones = person.getPhones();
  Phone head = phones.peek();
  assertSame(head, phones.poll());
  assertEquals( 1, phones.size() );`

  </div> </div> </div> </div> </div>