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

Question

I have two tables, DVD and Contact.

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)

Mordred

Agree with Bozho's answer, curious to know why you want to convert it to a concrete class, - what was your need to do that? – Boarish

Answer 1

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

Alphabetical

HashSet does provide extra methods than Set, an example is the clone() method. I've ran into this problem today and need to manually create my clone now... – Beachhead

Answer 2

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);

Heliozoan

This should be the correct answer as it addresses the lack of clone() pointed out in another comment. – Chifforobe

Answer 3

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.

Ochoa

Answer 4

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)

Cleodel

NONO a dvd can only be rent by one and only one user! – Mordred

@vinise One user at a given time, yes. But if it can only be rented by one user ever, then you misunderstood the concept of renting something. – Cleodel

yes :) don't worry its a very very specific case a dvd can be rent by a user and when the user bring it back the field contact (on dvd's row) is set to null ans then can be rent by an other user :) – Mordred