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 hashCode; 
    } 
    // the constructor, equals, and toString methods follow 
}

A StringBuffer cuts down on the number of objects created, since String concatenation is expensive. The code could be improved by saving the hash code in a private variable and returning that value in subsequent method calls; this way, the hash code is calculated only once in the life of the instance.