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From Dipanjan Laha <dipanja...@gmail.com>
Subject Re: [collections] Issues with MultiMap generics and need of a MultiTrie
Date Wed, 26 Feb 2014 09:25:39 GMT
Hi Thomas,

This sounds great. Moving MultiKeyMap to the new package does sound like
the way to go ahead. I will start with the implementation of the interface
and the MultiValuedHashMap. I should be able to submit a patch with a basic
implementation and some test cases by the end of this week. I can then
modify and incorporate changes as per your review and suggestions.

Regards
Dipanjan


On Wed, Feb 26, 2014 at 2:32 PM, Thomas Neidhart
<thomas.neidhart@gmail.com>wrote:

> Hi Dipanjan,
>
> I was thinking about a name for the new interface, but I actually like your
> proposal of MultiValuedMap.
>
> For the package, I think we can stick with multimap, and at some point we
> could also move the MultiKeyMap there, which would be logical imho.
>
> The implementation names are also sound.
>
> Thomas
>
>
>
>
> On Wed, Feb 26, 2014 at 9:28 AM, Dipanjan Laha <dipanjan21@gmail.com>
> wrote:
>
> > Hi Thomas,
> >
> > It would be great if we can start the discussion on the new interface for
> > MultiMap and a new package for the implementations as suggested by you.
> > Then I'll be able to put some code together for the same.
> >
> > IMO we can have
> >
> > 1. New Interface for MultiMap with the name MultiValuedMap or MultiValMap
> >  (as MultiValueMap is already the existing implementing class).
> > 2. New package for the implementations:
> > org.apache.commons.collections.multimap or
> > org.apache.commons.collections.multivaluedmap
> > 3. Implementation names like : MultiValuedHashMap, MultiValuedTreeMap etc
> >
> > Please let me know of your thoughts on these.
> >
> > Regards
> > Dipanjan
> >
> > On Sun, Feb 23, 2014 at 8:36 PM, Dipanjan Laha <dipanjan21@gmail.com>
> > wrote:
> >
> > > Hi Thomas,
> > >
> > > Thanks for your feedback. I created an improvement request in Jira for
> > the
> > > same (https://issues.apache.org/jira/browse/COLLECTIONS-508 ) as I
> > > thought it could be better tracked there. Sorry for the duplication in
> > the
> > > mail list and Jira. I have also attached a patch in Jira where I have
> > > modified the existing MultiMap interface and the MultiValueMap
> > > implementation and their test cases. I agree that it would break
> backward
> > > compatibility and we should go with your suggestion of deprecating the
> > > existing ones and design fresh interfaces for the same. The patch is
> > just a
> > > sample implementation to demonstrate the issue and is far from being
> > > complete in terms of documentation and test cases. I am also attaching
> > the
> > > patch here for your reference.
> > >
> > > Please go through the patch and also let me know of your thoughts on
> how
> > > we should proceed with the new interface and package structure. I'll be
> > > happy to change and redirect the implementation as per your
> suggestion. I
> > > am new to Apache Commons, but with some guidance I should not have
> issues
> > > implementing them to start with.
> > >
> > > As for MultiTrie, as you mentioned, we can start with it once the new
> > > MultiMap has been finalized.
> > >
> > > Regards
> > > Dipanjan
> > >
> > >
> > >
> > >
> > > On Sun, Feb 23, 2014 at 6:09 PM, Thomas Neidhart <
> > > thomas.neidhart@gmail.com> wrote:
> > >
> > >> On 02/22/2014 02:00 PM, Dipanjan Laha wrote:
> > >> > Hello,
> > >>
> > >> Hi Dipanjan,
> > >>
> > >> > Recently I had the need of using a MultiMap in one of my projects.
I
> > >> found
> > >> > that commons collection already has a MultiMap interface and an
> > >> > implementation.
> > >> >
> > >> > While using the same, I found that the MultiMap interface  has
> methods
> > >> that
> > >> > are not strongly typed even though the interface supports generics.
> > For
> > >> > example if I have a MultiMap like so
> > >> >
> > >> > MultiMap<String, User> multiMap = new MultiValueMap<String,
User>();
> > >> >
> > >> > where User is a custom  Class, then the get(key) method would return
> > me
> > >> an
> > >> > Object which I would need to cast to a Collection like so
> > >> >
> > >> > Collection<User> userCol = (Collection<User>) multiMap.get(key);
> > >> >
> > >> > I understand that this limitation comes from that fact that the
> > MultiMap
> > >> > extends IterableMap which in turn extends Map and other interfaces.
> > >> Hence
> > >> > the MultiMap cannot have a get method which returns a Collection
> > >> instead of
> > >> > Object as that would mean extending IterableMap with the Generics
> set
> > >> to be
> > >> > <K,Collection<V>>. In that case the put method's signature
would
> > become
> > >> >
> > >> > public Collection<V> put(K key, Collection<V> value);
> > >> >
> > >> > which we do not want.The same problem would arise with other methods
> > as
> > >> > well, ex: containsValue method.
> > >> >
> > >> > My proposal is why carry on the signatures of a Map and put it on
> > >> MultiMap.
> > >> > Where as I do agree that it is a Map after all and has very similar
> > >> > implementation and functionality, it is very different at other
> > levels.
> > >> And
> > >> > even though the MultiMap interface supports generics, the methods
> are
> > >> not
> > >> > strongly typed, which defeats the purpose of having generics. So why
> > >> can't
> > >> > we have a separate set of interfaces for MultiMap which do not
> extend
> > >> Map.
> > >> > That way we can have strongly typed methods on the MultiMap.
> > >>
> > >> The MultiMap interface as it is right now is flawed, and should have
> > >> been cleaned up prior to the 4.0 release imho (and I regretted it
> > >> already before your post).
> > >>
> > >> As you correctly pointed out, the problem comes from the fact that it
> > >> extends Map<K, Object> which leads to problems once generics have
been
> > >> introduced (before it did not matter that much as you had to cast
> > >> anyway, as it is also documented in the javadoc).
> > >>
> > >> One mitigation for this was the introduction of this method to
> > >> MultiValueMap, but it is clearly not enough:
> > >>
> > >>  public Collection<V> getCollection(Object key)
> > >>
> > >>
> > >> Unfortunately, it is not easy to fix this now after collections 4.0
> has
> > >> been released. We need to keep backwards compatibility, but we could
> do
> > >> the following:
> > >>
> > >>  * deprecate the existing interfaces/classes:
> > >>    - MultiMap
> > >>    - MultiValueMap
> > >>
> > >>  * design a new, clean interface (by not extending Map)
> > >>  * add new package multimap with concrete implementations for
> different
> > >>    types of maps (right now only hashmaps are supported)
> > >>
> > >> > Please let me know your thoughts on this. I can submit a patch for
> > these
> > >> > changes based on your feedback. One more thing, I also am in need
> of a
> > >> > MultiTrie which is currently not there. I am implementing the same
> by
> > >> > wrapping PatriciaTrie. Now I am a bit confused here as, if I make
> the
> > >> > MultiTrie interface on the lines of MultiMap, it would have the same
> > >> > limitations. In that case I was planning to have a separate set of
> > >> > interfaces for MultiTrie which does not extend any other interface.
> > And
> > >> in
> > >> > case, we do change the MultiMap interface to be independent of Map,
> > then
> > >> > MultiTrie can extend MultiMap. Please let me know your thoughts on
> > this
> > >> as
> > >> > well as I am implementing the same for my project right now and
> would
> > >> like
> > >> > to contribute it back to the commons collection.
> > >>
> > >> Patches are always welcome, but we first need a decision in which
> > >> direction to go, see above.
> > >>
> > >> Regarding the MultiTrie:
> > >>
> > >> Indeed, it is the same problem, so it should go hand in hand with the
> > >> revamp of the MultiMap interface.
> > >>
> > >> Thomas
> > >>
> > >> ---------------------------------------------------------------------
> > >> To unsubscribe, e-mail: dev-unsubscribe@commons.apache.org
> > >> For additional commands, e-mail: dev-help@commons.apache.org
> > >>
> > >>
> > >
> >
>

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