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Java Enterprise Best Practices
book

Java Enterprise Best Practices

by O'Reilly Java Authors
December 2002
Intermediate to advanced content levelIntermediate to advanced
288 pages
9h 46m
English
O'Reilly Media, Inc.
Content preview from Java Enterprise Best Practices

Know When to Use Compound Primary Keys

If you decide to write a compound primary key class for an entity bean, you must override the hashCode( ) and equals( ) methods. It is also common practice to override the toString( ) method for debugging purposes. Overriding hashCode( ) and equals( ) is important because the container uses these methods to compare primary keys and to look up cached entity bean instances.

Implementing the equals( ) method is pretty simple—all you have to do is compare various components of the primary key. On the other hand, implementing hashCode( ) is harder because a poorly implemented hashCode( ) method can slow down entity bean lookups, especially if there are a lot of instantiated beans. This happens because application servers use HashMap-type structures to store primary keys, and the performance of these structures relies on the fact that hashCode( ) returns different values for different objects.

In most cases, the implementation of hashCode( ) should depend on the data that the primary key contains. However, there are a few generic algorithms that work well. The one shown in Example 2-3 is taken from the java.util.List.hashCode( ) method. This algorithm simply adds all hashcodes of the primary key fields, multiplying each intermediate result by 31 (so that it’s not a simple sum of hashcodes).

Example 2-3. Compound primary key
public class CompoundPK implements Serializable { private String str1; private String str2; private int int1; private ...
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Publisher Resources

ISBN: 0596003846Supplemental ContentErrata Page