Relationship that is both ordered and rejects duplicates

breako · **Joined:** Mon Jul 24, 2006 8:43 am **Posts:** 160

Hi,
I need my assocations in my POJOs to be both ordered and to reject duplicates. If an entity is attempted to be added to a association relationship which is already in the association relationship, I wish for it to be ignored.

The List interface helps me for the ordered part but does not for duplication rejection requirement.

I notice as well that hibernate will always use it's own implementation for the association i.e. PersistentBag anyway so there is not point me specifying a smart custom List implementation for the association.

So the only way I can think to reject duplicates is to put in some extra logic into the POJO? I don't want to have to do this, as I want to keep the POJOs as simple and lightweight as possible,
Any ideas?

andydale · **Posted:** Fri Mar 02, 2007 7:43 am

Hi,

The only type of Object used in relationships that is easily able (no code required) to reject duplicates is a java.util.Set, which by design does not allow duplicates, but i think it is not possible to enforce ordering (i have not attempted it).

But maybe you could attempt to create it as so:

Code:

private Set<Entity> mEntities = new LinkedHashSet<Entity>();

or

Code:

private Set<Entity> mEntities = new SortedSet<Entity>();

but i have only used HashSet (you might be able to use it in conjunction with an @OrderBy).

Cheers,

Andy

andydale · **Posted:** Fri Mar 02, 2007 7:57 am

just had a quick look over the documentation and SortedSet is suppoerted by hibernate.

look at section 6.6 here (http://www.hibernate.org/hib_docs/reference/en/html/collections.html) for further info

Cheers,

Andy

breako · **Joined:** Mon Jul 24, 2006 8:43 am **Posts:** 160

andydale wrote:

just had a quick look over the documentation and SortedSet is suppoerted by hibernate.

look at section 6.6 here (http://www.hibernate.org/hib_docs/reference/en/html/collections.html) for further info

Cheers,

Andy

Thanks for getting back to me Andy and I agree that is one way of doing it. However then I loose all the index api's, i.e. the get(index i) etc. api's which are really handy. It doesn't look like hibernate facilitate you writing your own List implementation which is a drawback.
Anybody any ideas or comments?
Cheers

Fireblaze · **Joined:** Fri Mar 09, 2007 12:41 pm **Posts:** 3

Keeping the POJO as simple as possible, there for only using the Java standard library. The solution is:

Code:

private List<Entity> list = new LinkedList<Entity>();

public void add(Entity entity) {
  if(list.contains(entity)) {
     return;
  }
  list.add(entity);
}

Or you could extract the code into a helper class, leaving only one line of code and no extra complexity in your POJO.

Code:

 
class CollectionHelper {
    public static <E> void addNoDuplicates(Collection<E> first, E ... second) {
        addNoDuplicates(first, Arrays.asList(second));
    }

    public static <E> void addNoDuplicates(Collection<E> first, Collection<E> second) {
        for(E object: second) {
            if(!first.contains(object)) {
                first.add(object);
            }
        }
    }
}

private List<Entity> list = new LinkedList<Entity>();

public void add(Entity entity) {
    CollectionHelper.addNoDuplicates(list, entity);
}

breako · **Joined:** Mon Jul 24, 2006 8:43 am **Posts:** 160

Fireblaze wrote:

Keeping the POJO as simple as possible, there for only using the Java standard library. The solution is:

Code:

private List<Entity> list = new LinkedList<Entity>();

public void add(Entity entity) {
  if(list.contains(entity)) {
     return;
  }
  list.add(entity);
}

Or you could extract the code into a helper class, leaving only one line of code and no extra complexity in your POJO.

Code:

 
class CollectionHelper {
    public static <E> void addNoDuplicates(Collection<E> first, E ... second) {
        addNoDuplicates(first, Arrays.asList(second));
    }

    public static <E> void addNoDuplicates(Collection<E> first, Collection<E> second) {
        for(E object: second) {
            if(!first.contains(object)) {
                first.add(object);
            }
        }
    }
}

private List<Entity> list = new LinkedList<Entity>();

public void add(Entity entity) {
    CollectionHelper.addNoDuplicates(list, entity);
}

Hi,
Thanks for that, I thought about that and agree it will work.
It will just result in an explosion of APIs on my pojo as I must have an accessor and mutator for every association and if I now also have to extra addXXX() and removeXXX() methods it will result in an explosion of APIs as I have quite a lot of relationship associations.
It's a pity there isn't a
@NoDuplicates annotation.
Thanks for your comments.

emmanuel · **Posted:** Tue Mar 13, 2007 6:47 pm

Note that if you exclude duplicates, indexes does not make sense, that's why the Set API does not have the notion in the first place.