Example 4.5. Application Identity Class

/**
 * Persistent class using application identity.
 */
public class Magazine
{
    private String isbn;    // primary key field
    private String title;   // primary key field

    /**
     * Equality must be implemented in terms of primary key field
     * equality, and must use instanceof rather than comparing 
     * classes directly.
     */
    public boolean equals (Object other)
    {
        if (other == this)
            return true;
        if (!(other instanceof Magazine))
            return false;

        Magazine mag = (Magazine) other;
        return ((isbn == null && mag.isbn == null)
            || (isbn != null && isbn.equals (mag.isbn)))
            && ((title == null && mag.title == null)
            || (title != null && title.equals (mag.title)));
    }
     
    /**
     * Hashcode must also depend on primary key values.
     */
    public int hashCode ()
    {
        return ((isbn == null) ? 0 : isbn.hashCode ())
            + ((title == null) ? 0 : title.hashCode ())
            % Integer.MAX_VALUE;
    }

    // rest of fields and methods omitted


    /**
     * Application identity class for Magazine.
     */
    public static class MagazineId
    {
        static
        {
            // register Magazine with the JVM
            Class c = Magazine.class;
        }
    
        // each primary key field in the Magazine class must have a
        // corresponding public field in the identity class
        public String isbn;
        public String title;

        /**
         * Default constructor requirement.
         */
        public MagazineId ()
        {
        }
    
        /**
         * String constructor requirement.
         */
        public MagazineId (String str)
        {
            int idx = str.indexOf (':');
            isbn = str.substring (0, idx);
            title = str.substring (idx + 1);
        }
    
        /**
         * toString must return a string parsable by the string constructor.
         */
        public String toString ()
        {
            return isbn + ":" + title;
        }
    
        /**
         * Equality must be implemented in terms of primary key field
         * equality, and must use instanceof rather than comparing 
         * classes directly (some JDO implementations may subclass JDO
         * identity class).
         */
        public boolean equals (Object other)
        {
            if (other == this)
                return true;
            if (!(other instanceof MagazineId))
                return false;
    
            MagazineId mi = (MagazineId) other;
            return ((isbn == null && mi.isbn == null)
                || (isbn != null && isbn.equals (mi.isbn)))
                && ((title == null && mi.title == null)
                || (title != null && title.equals (mi.title)));
        }
     
        /**
         * Hashcode must also depend on primary key values.
         */
        public int hashCode ()
        {
            return ((isbn == null) ? 0 : isbn.hashCode ())
                + ((title == null) ? 0 : title.hashCode ())
                % Integer.MAX_VALUE;
        } 
    }
}