Enterprise JavaBeans, Third Edition

Chapter 15. Design Strategies

The previous 14 chapters have presented the core EJB technology. What’s left is a grab bag of miscellaneous issues: how to solve particular design problems, how to work with particular kinds of databases, and topics of that nature.

Hash Codes in Compound Primary Keys

Chapter 11 discusses the necessity of overriding the Object.hashCode() and Object.equals() methods in the primary key class of an entity bean. With complex primary keys that have several fields, overriding Object.equals() is fairly trivial. However, overriding Object.hashCode() is more complicated, because an integer value that can serve as a suitable hash code must be created from several fields.

One solution is to concatenate all the values into a String and use the String object’s hashCode() method to create a hash code value for the whole primary key. The String class has a decent hash code algorithm that generates a fairly well-distributed and repeatable hash code value from any set of characters.

The following code shows how to create such a hash code for a hypothetical primary key:

public class HypotheticalPrimaryKey implements java.io.Serializable { public int primary_id; public short secondary_id; public java.util.Date date; public String desc; public int hashCode() { StringBuffer strBuff = new StringBuffer(); strBuff.append(primary_id); strBuff.append(secondary_id); strBuff.append(date); strBuff.append(desc); String str = strBuff.toString(); int hashCode = str.hashCode(); return ...