Difference between Set and Bag in Hibernate

up vote1down votefavorite 1	What is the main difference between the Set and Bag collections in Hibernate? In what scenarios should we use Set and Bag? hibernate share|improve this question edited Oct 1 at 18:27 hopper 2,84731225 asked Dec 11 '12 at 1:35 Srinivasan 1,29252345
	1   Read the documentation of List and Set interfaces. –  AVD Dec 11 '12 at 1:37 add comment

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From the Hibernate reference: Bags are the worst case since they permit duplicate element values and, as they have no index column, no primary key can be defined. Hibernate has no way of distinguishing between duplicate rows. And also: There is a particular case, however, in which bags, and also lists, are much more performant than sets. For a collection with inverse="true", the standard bidirectional one-to-many relationship idiom, for example, we can add elements to a bag or list without needing to initialize (fetch) the bag elements. share|improve this answer answered Dec 11 '12 at 8:53 Atropo 3,305717

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A  is an unordered collection, which can contain duplicated elements. That means if you persist a bag with some order of elements, you cannot expect the same order retains when the collection is retrieved. There is not a “bag” concept in Java collections framework, so we just use ajava.util.List corresponds to a . A  is similar to  except that it can only store unique objects. That means no duplicate elements can be contained in a set. When you add the same element to a set for second time, it will replace the old one. A set is unordered by default but we can ask it to be sorted. The corresponding type of a in Java is java.util.Set. Examples Mapping  name="employees" table="employee" inverse="true" lazy="true" fetch="select"> name="department_id" not-null="true" />

class="net.viralpatel.hibernate.Employee" />

Mapping 

   name="employees" table="employee"
                inverse="true" lazy="true" fetch="select">
            
                 name="employee_id" not-null="true" />
            

class="net.viralpatel.hibernate.Employee" />

Thus, both are mapped exactly same way in hbm file. But differs only in the way it handles duplicate records.

Source: Hibernate One to Many XML Tutorial

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answered Dec 12 '12 at 16:06 Viral Patel 2,022918

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Both are unordered collections. Bags allow duplicates. Sets do not. share|improve this answer answered Dec 11 '12 at 1:39 Justin Niessner 132k6198321