Sets in Hibernate suffer because you can store an object before it was updated and Hibernate will update it.

E.g. If say

    Person { 
       id: Integer, 
       name: String
    }

Let's say I have some entity that stores Persons as HashSet. If I add them before saving, their hash set is taken without Id.

Id can be automatically generated by Hibernate on save/create.

After save the Id now has a new value. I.e. its hash differs from when it was saved.

Boom. You have HashSet with Zombie entries. That are technically that you can't fetch actually.