Re: Indexing by multiple keys
Daniel Pitts wrote:
You can use whatever keys you want in a HashMap.
You can also use multiple hash-maps if you're keyspaces aren't disjoint.
Map<String, Person> byName;
Map<SocialSecurityNumber, Person> bySsn;
Map<Color, List<Person>> byFavoriteColor;
Great examples.
For that last I proffer
Map <Color, Set <Person>> byFavoriteColor;
to prevent duplicate entries in the target collection.
A word to the wise - this technique increase the programmer's responsibility
to clean up references. It's all too easy to leave a 'Person' reference
buried in 'byFavoriteColor' that you removed from 'byName' and 'bySsn'.
With JPA, @Entity and judicious use of @OneToMany and siblings to inject
member collections, you mitigate the risk of reference retention because you
don't maintain multiple long-lived independent structures.
EntityManager and its related mechanisms provide useful resource and entity
object management. The programmer can't abdicate responsibility, but the JPA
provides methods that simplify control.
For example you could create the cited 'Map's at need rather than as permanent
edifices. You do a little query, retrieve one or another useful collection of
entities expressing one or another useful relationship, do something useful
with those objects, then pass the objects out of scope, mayhap closing an
entity manager on the way out. There's less chance of packratting with that
idiom.
JPA managers also unify managed instances so that you don't get entity bloat.
--
Lew