Problem while reordering a managed collection

Question

Hello,
I'm trying to implement a feature that allows users to freely
re-arrange items into a
managed collection, using Drag and Drop. I'm using a
TileList object, which already has built-in support for Drag and
Drop, provided by Flex.
A FDS destination (implemented by a Java class) is used to
fill the collection with data (the fill method gets called).
The problem is when the user tries to re-arrange the items in
the list, using DnD. The Flex client sends to FDS a "delete"
operation message, followed by a "create" one. I don't really want
to do a "delete" and a "create" on the server side / (FDS's managed
collection and the uunderlying database), as there's no really a
delete and a create, it's rather a "move" sort of thing that should
happen.
What I want FDS to do is to ignore the list of changes
("delete" and "create") on it's sync method, somehow manage to
re-arrange it's managed collection, and send an OK back to the
client, without doing any auto-refresh. that is, I don't need any
database trip to happen here for now.
I'm having trouble making these to happen. I tried various
approaches so far, with no success. Any ideas?
Thanks,
Robert

Answer 1

don,
Did you realise that your IntegerSet class isn't very useful?
Why? you ask... Well the only change to the behaviour of java.util.HashSet is that your [add method|http://java.sun.com/javase/6/docs/api/java/util/HashSet.html#add(E)] throws IntegerAleadyExistsException, instead of just returning false.
Rule of thumb: If you can return a meaninful value (instead of throwing an exception) then that's the way to go... because it simplifies the calling code.
the return value solution ...
  if ( !set.add(item) ) {
    System.err.println("Item "+item+" allready exists. Guess again.");
... verses the exception solution ...
  try {
    set.add(item);
  } catch (ItemAleadyExistsException e) {
    System.err.println("Item "+item+" allready exists. Guess again.");
  }Which do you think is "cleaner" easier to maintain?
Also: HashSet is generic (meaning that it can be used to hold a set of any-type) whereas your class is limited to holding Integers... so it's much less broadly "useful".
You're writing this class for the learning experience, not for the usefulness of it, right? Cool... but it might be good for you (in the long run) to know that such a "limited" class is... ummm... a waste of code; and you should think about when to throw an exception, and when not to. Exception handling is an important and much-over-looked aspect of good system design.
Cheers. Keith.