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Chapter 6. Collection mapping

6.1. Persistent collections

Hibernate requires that persistent collection-valued fields be declared as an interface type. For example:
public class Product {
    private String serialNumber;
    private Set parts = new HashSet();
    public Set getParts() { return parts; }
    void setParts(Set parts) { = parts; }
    public String getSerialNumber() { return serialNumber; }
    void setSerialNumber(String sn) { serialNumber = sn; }
The actual interface might be java.util.Set, java.util.Collection, java.util.List, java.util.Map, java.util.SortedSet, java.util.SortedMap or anything you like ("anything you like" means you will have to write an implementation of org.hibernate.usertype.UserCollectionType.)
Notice how the instance variable was initialized with an instance of HashSet. This is the best way to initialize collection valued properties of newly instantiated (non-persistent) instances. When you make the instance persistent, by calling persist() for example, Hibernate will actually replace the HashSet with an instance of Hibernate's own implementation of Set. Be aware of the following errors:
Cat cat = new DomesticCat();
Cat kitten = new DomesticCat();
Set kittens = new HashSet();
kittens = cat.getKittens(); // Okay, kittens collection is a Set
(HashSet) cat.getKittens(); // Error!
The persistent collections injected by Hibernate behave like HashMap, HashSet, TreeMap, TreeSet or ArrayList, depending on the interface type.
Collections instances have the usual behavior of value types. They are automatically persisted when referenced by a persistent object and are automatically deleted when unreferenced. If a collection is passed from one persistent object to another, its elements might be moved from one table to another. Two entities cannot share a reference to the same collection instance. Due to the underlying relational model, collection-valued properties do not support null value semantics. Hibernate does not distinguish between a null collection reference and an empty collection.
Use persistent collections the same way you use ordinary Java collections. However, please ensure you understand the semantics of bidirectional associations (these are discussed later).