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 ...