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,

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.