Java, Hibernate java.lang.ClassCastException: org.hibernate.collection.PersistentSet cannot be cast to java.util.HashSet

I have two tables, DVD and Cont开发者_JS百科act.

A DVD can be rented to a contact and a contact can rent many DVD's.

The many to one link (dvd-->contact) works fine.

But the other way fails: (contact-->dvd)

This is the contact mapping:

<set name="dvds" inverse="true">
   <key column="contactId"/>
   <one-to-many class="Dvd"/>
</set>

Here is setter getter for Contact:

private Set<Dvd> dvds = new HashSet<Dvd>();

public Set<Dvd> getDvds(){
   return dvds;
}
public void setDvds(Set<Dvd> dvds){
   this.dvds=dvds;
}

When I try to get the DVD rented from a contact with this:

HashSet<Dvd> tt = (HashSet<Dvd>)dds;

I get an Exception:

java.lang.ClassCastException: org.hibernate.collection.PersistentSet 
cannot be cast to java.util.HashSet

What does the Exception mean and how do I fix it?

Edit: This solved my problem:

.setResultTransformer(Criteria.DISTINCT_ROOT_ENTITY)

You don't need to cast to HashSet. It is a Set, and it does not provide any additional methods. So just don't cast.

This is a general rule when working with collections - don't refer to them with their concrete classes (unless you really need to). Use List and Set, rather than ArrayList and HashSet

Don't try to cast the Set dds into HashSet. Hibernate uses its own implementation of the Set interface called PersistentSet which does not derive from HashSet and hence the casting throws a ClassCastException. Either use it through the Set interface, or create a new HashSet using its constructor (in which case your changes to the set will not be reflected in Hibernate automatically).

Set<Dvd> tt = dds;

OR

HashSet<Dvd> tt = new HashSet<Dvd>(dds);

Abhinav Sarkar's answer is correct of course, but there is also a mistake in your modeling.

The relationship between DVD and Contact is Many-To-Many, not Many-To-One (otherwise each DVD would be unique to one single customer)

I've come across this problem recently. I was able to eradicate the casting problem.

List<Object> listObject = Arrays.asList(ListFromHibernate.toArray());

Then you can get the objects by casting the objects in List, say.

MyObject x = (MyObject) listObject.get(1);

PS: It's freaking 2013.