equals() and hashcode() important

Normally, most Java objects provide a built-in equals() and hashCode() based on the object's identity; so each new() object will be different from all others.

This is generally what you want in ordinary Java programming. And if all your objects are in memory, this is a fine model. Hibernate's whole job, of course, is to move your objects out of memory. But Hibernate works hard to prevent you from having to worry about this.

Hibernate uses the Hibernate session to manage this uniqueness. When you create an object with new(), and then save it into a session, Hibernate now knows that whenever you query for an object and find that particular object, Hibernate should return you that instance of the object. And Hibernate will do just that.

However, once you close the Hibernate session, all bets are off. If you keep holding onto an object that you either created or loaded in a Hibernate session that you have now closed, Hibernate has no way to know about those objects. So if you open another session and query for "the same" object, Hibernate will return you a new instance. Hence, if you keep collections of objects around between sessions, you will start to experience odd behavior (duplicate objects in collections, mainly).

The general contract is: if you want to store an object in a List, Map or a Set then it is an requirement that equals and hashCode are implemented so they obey the standard contract as specified in the documentation.

 

http://hibernate.org/109.html