I used Set to store many-to-one association, but I happened a boring problem. Although it is a j2se Set problem, it occurs in using hibernate.
My business logic is: Add a new detail Item to parent, save it to database. If any dababase exception(HibernateException) orrur, break the association.
So my code is like this:

But I found the Set:children would not remove child correctly. I think the reason is diffierent hashCode between child instance adding to Set and child instance removing from set.
Did anyone happen this problem before? Please share your solution?

Again, If I add a batch of children, then store at last. If occur HibernateException, I call method session.refresh to reload correct data from DB. Is it a good solution?

Your equals/hashCode implementation has to be such that it does not change during the object lifetime in a set. Look at http://www.hibernate.org/109.html for more discussion on your options.

Yes, I know the basic principle of equals and hashCode. My hashCode method in Child class includes field of Parent. Because two new instance with id==null and other fields are both same expect Parent field. So it exactly different hashCode before association parent instance and after association parent one. I think perhaps call child.setParent() first before add child to Set will be a better way. But it is not a root way to solve problem. Because after successing insert child to DB, field id of Child will be modified. If I need remove child from Set after successing insert into DB, it will also crash.
I read the article and http://www.hibernate.org/109.166.html
point out this problem.
So I don't think maintain same hashCode during the object lifetime in a set is a easy thing. And is there any common solution aim to changing hashCode during object in a Set?