I have a couple books on design patterns but I'm still a bit stumped on
how to 'best' handle this situation...
For the sake of explaining this I'm using an example of "Stores." The
example of stores isn't perfect but it's easier to picture than my real
life example.
What I want to be able to is pull out a List of "items" from a Store
object that will never change for the life of the web application (I
know in real life this wouldn't be the case for Store items). Then I
also have subclasses of Store - "Target" and "Walmart" - where I need to
pull out a List of "specials" and these unique Lists for Walmart and
Target will not change for those subclasses.
Now each of the above classes only needs to be instantiated once so they
can be all singletons and the Lists only need to be populated once. It
gets a bit tricky since the creation of the object instances need to be
thread safe and you can't really make the superclass (Store) a singleton
if you plan to subclass it. So what I've done is this...
public abstract class Store {
private static Collection items = null;
private Collection specials = null;
static {
if ( items == null ) {
synchronized( Store.class) {
if ( items == null ) {
items = //get List of Items from somewhere
}
}
}
}
public static Collection getItems() {
return items;
}
public Collection getSpecials() {
return specials;
}
}
public class Walmart extends Store {
static private Target _instance;
private Walmart() {
specials = //get specials unique for Walmart
}
public static Walmart getInstance() {
if (_instance == null) {
synchronized(Walmart.class) {
if (_instance == null) {
_instance = new Walmart();
}
}
}
return _instance;
}
}
public class Target extends Store {
//same thing like above but gets specials unique for Target
}
What I then have is a factory that figures out what Store to give me so
in my code I can just call:
Store s = SomeFactory.getStore() //and SomeFactory returns to me the
correct type of store.
The above seems to work ok, but is it a crappy design and are there
still some multithreading issues that could screw things up?
Thanks,
--
Rick Reumann
---
You are currently subscribed to jdjlist as: [EMAIL PROTECTED]
To unsubscribe send a blank email to [EMAIL PROTECTED]
http://www.sys-con.com/fusetalk
