2.8. Collections

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Naturally Hibernate also allows to persist collections.
These persistent collections can contain almost any other Hibernate type, including: basic types, custom types, components and references to other entities.
In this context, the distinction between value and reference semantics is very important.
An object in a collection might be handled with _value_ semantics (its life cycle being fully depends on the collection owner),
or it might be a reference to another entity with its own life cycle.
In the latter case, only the _link_ between the two objects is considered to be a state held by the collection.

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The owner of the collection is always an entity, even if the collection is defined by an embeddable type.
Collections form one/many-to-many associations between types so there can be:

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• value type collections
• embeddable type collections

• entity collections </div>

  Hibernate uses its own collection implementations which are enriched with lazy-loading, caching or state change detection semantics. For this reason, persistent collections must be declared as an interface type. The actual interface might be java.util.Collection, java.util.List, java.util.Set, java.util.Map, java.util.SortedSet, java.util.SortedMap or even other object types (meaning you will have to write an implementation of org.hibernate.usertype.UserCollectionType).

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  As the following example demonstrates, it’s important to use the interface type and not the collection implementation, as declared in the entity mapping.

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  Example 142. Hibernate uses its own collection implementations

  `@Entity(name = "Person")
  public static class Person {

  @Id
  private Long id;
  
  @ElementCollection
  private List<String> phones = new ArrayList<>();
  
  public List<String> getPhones() {
      return phones;
  }
  

  }

  Person person = entityManager.find( Person.class, 1L ); //Throws java.lang.ClassCastException: org.hibernate.collection.internal.PersistentBag cannot be cast to java.util.ArrayList ArrayList<String> phones = (ArrayList<String>) person.getPhones();`</pre> </div> </div> </div> </div>

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  It is important that collections be defined using the appropriate Java Collections Framework interface rather than a specific implementation. From a theoretical perspective, this just follows good design principles. From a practical perspective, Hibernate (like other persistence providers) will use their own collection implementations which conform to the Java Collections Framework interfaces.

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  The persistent collections injected by Hibernate behave like ArrayList, HashSet, TreeSet, HashMap or TreeMap, depending on the interface type.

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