CHAPTER 9 Working with objects Understanding equals() and (Web server info)
CHAPTER 9 Working with objects Understanding equals() and hashCode() Before we show you how to implement your own equality routine. we have to bring two important points to your attention. First, in our experience, many Java developers never had to override the equals() and hashCode() methods before using Hibernate (or Java Persistence). Traditionally, Java developers seem to be unaware of the intricate details of such an implementation. The longest discussion threads on the public Hibernate forum are about this equality problem, and the blame is often put on Hibernate. You should be aware of the fundamental issue: Every object-oriented programming language with hash-based collections requires a custom equality routine if the default contract doesn t offer the desired semantics. The detached object state in a Hibernate application exposes you to this problem, maybe for the first time. On the other hand, you may not have to override equals() and hashCode(). The identity scope guarantee provided by Hibernate is sufficient if you never compare detached instances that is, if you never put detached instances into the same Set. You may decide to design an application that doesn t use detached objects. You can apply an extended persistence context strategy for your conversation implementation and eliminate the detached state from your application completely. This strategy also extends the scope of guaranteed object identity to span the whole conversation. (Note that you still need the discipline to not compare detached instances obtained in two conversations!) Let s assume that you want to use detached objects and that you have to test them for equality with your own routine. You can implement equals() and hash- Code() several ways. Keep in mind that when you override equals(), you always need to also override hashCode() so the two methods are consistent. If two objects are equal, they must have the same hashcode. A clever approach is to implement equals() to compare just the database identifier property (often a surrogate primary key) value: public class User { … public boolean equals(Object other) { if (this==other) return true; if (id==null) return false; if ( !(other instanceof User) ) return false; final User that = (User) other; return this.id.equals( that.getId() ); } public int hashCode() { return id==null ?
We recommend you use shared web hosting services, because many users agree that it is cheap, reliable and customer-satisfying webhost.