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    <p>Using the <code>id</code> of an entity is not a good idea because transient entities don't have an id yet (and you still want a transient entity to be potentially equal to a persistent one).</p> <p>Using all properties (apart from the database identifier) is also not a good idea because all properties are just not part of the identity.</p> <p>So, the preferred (and correct) way to implement equality is to use a <strong>business key</strong>, as explained in <em>Java Persistence with Hibernate</em>:</p> <blockquote> <h3>Implementing equality with a business key</h3> <p>To get to the solution that we recommend, you need to understand the notion of a business key. A business key is a property, or some combination of properties, that is unique for each instance with the same database identity. Essentially, it’s the natural key that you would use if you weren’t using a surrogate primary key instead. Unlike a natural primary key, it isn’t an absolute requirement that the business key never changes—as long as it changes rarely, that’s enough.</p> <p>We argue that essentially every entity class should have some business key, even if it includes all properties of the class (this would be appropriate for some immutable classes). The business key is what the user thinks of as uniquely identifying a particular record, whereas the surrogate key is what the application and database use.</p> <p>Business key equality means that the equals() method compares only the properties that form the business key. This is a perfect solution that avoids all the problems described earlier. The only downside is that it requires extra thought to identify the correct business key in the first place. This effort is required anyway; it’s important to identify any unique keys if your database must ensure data integrity via constraint checking.</p> <p>For the User class, <code>username</code> is a great candidate business key. It’s never null, it’s unique with a database constraint, and it changes rarely, if ever:</p> <pre><code> public class User { ... public boolean equals(Object other) { if (this==other) return true; if ( !(other instanceof User) ) return false; final User that = (User) other; return this.username.equals( that.getUsername() ); } public int hashCode() { return username.hashCode(); } } </code></pre> </blockquote> <p>Maybe I missed something but for an Address, the business key would typically be made of the street number, the street, the city, the postal code, the country. I don't see any problem with that.</p> <p>Just in case, <a href="http://community.jboss.org/wiki/EqualsandHashCode" rel="noreferrer">Equals And HashCode</a> is another interesting reading.</p>
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    1. USPascal Thivent
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    1. COthanks for your input. the problems i saw with it (guess i did not made myself clear enough) is: a) the hashcode changes as the user enters more data b) when the user presses the 'add address' button (just as an example) all fields are null (in your User example, the username is not yet given). I will have to read about the hashCode(), perhaps my memory is wrong (and a changing hashCode is not as evil as i remember) thanks
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    2. COYou may want to replace "return this.username.hashCode()" to "return this.getUserName().hashCode()" and "return this.username.equals(that.getUserName())" to "return this.getUserName().equals(that.getUserName())". It's a better practice.
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      1. USRobert Durgin
 

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