What is the difference between using a Set or a Collection for @OneToMany or @ManyToMany properties on my hibernate entity objects?
Does Hibernate map things differently depending on which one you choose?
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Can you use collection? I thought collection is the generic term but you have to decide between Bag, Set, SortedSet, etc. ..Thorsten– Thorsten2009-02-02 18:44:47 +00:00Commented Feb 2, 2009 at 18:44
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Looking at the answers and the direction I was thinking .. where are you using the set or the collection .. in the mapping xml or in your java code to retrieve the results of a query?Thorsten– Thorsten2009-02-02 18:59:50 +00:00Commented Feb 2, 2009 at 18:59
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I'm using hibernate annotations so I'll have @OneToMany private Set<Foo> foo; or @OneToMany private Collection<Foo> foo; Both seem to work but I'm trying to understand what if any difference there is.timdisney– timdisney2009-02-02 19:06:35 +00:00Commented Feb 2, 2009 at 19:06
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5 Answers
In the context of hibernate, following is the scenario under which you would use Set instead of Collection: -
"From Order as orders left fetch join orders.orderLineItems as orderLineItems ORDER BY orders.id DESC"
It returns duplicates so use the hash set to remove them.
Query query = session.getNamedQuery("OrdersAndLoadLineItems");
Set set = new LinkedHashSet();
set.addAll(query.list());
return set;
This is taken from Hibernate FAQ link is http://www.hibernate.org/117.241.html
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Collection is an Interface, cannot be instantiated. Set is also an Interface.
As such, it doesn't matter what you use, as long as the instantiated object that you use is compatible with those.
So normally, you would do something like this:
private Set parts = new HashSet();
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I don't know the difference, but if you use Set you can fetch multiple bags in JPA, but if you use List, for instance, you cannot fetch multiple bags in a query.
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