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From Dipanjan Laha <dipanja...@gmail.com>
Subject Re: [collections] Issues with MultiMap generics and need of a MultiTrie
Date Sun, 02 Mar 2014 16:46:38 GMT
Hi Thomas,

I have implemented the MultiValuedMap interface, the MultiValuedHashMap and
a MultiValuedHashMapTest as per the discussions. I haven't completed the
documentation yet. If the implementations look fine, I will add the
remaining documentations.

A few more points regarding the implementation

1. I have added a few methods to the MultiValuedMap interface which were
not there in the MultiMap. I think they would be a good addition to the
interface IMHO. They are
    boolean containsValue(Object key, Object value);
    int totalSize();
    void putAll(MultiValuedMap<? extends K, ? extends V> map);
2. I have added an AbstractMultiValuedMapDecoractor on the lines
of AbstractMapDecorator, which can be extended by other MultiValuedMap
implementations like say a MultiValuedTreeMap
3. I have created MultiValuedGet and MultiValuedPut to honor the Get/Put
split concepts. It was not possible for MultiValuedMap to extend the Get &
Put directly due to the limitations I had mentioned in my earlier mail.
4. I have marked the incomplete documentations with TODO tags.

PFA the patch for the new Classes. Please go through the implementation and
let me know if I missed some thing or if things need to be done in some
other way.

Regards
Dipanjan



On Wed, Feb 26, 2014 at 8:04 PM, Dipanjan Laha <dipanjan21@gmail.com> wrote:

> Thanks for pointing this out. However, implementing Get & Put directly
> would pose the following problems.
>
> If interface MultiValuedMap<K,V> extends Get<K,Collection<V>>
>
> the method "values" would be forced to have a signature of
>
> Collection<Collection<V>> values()
>
> whereas we would want
>
> Collection<V> values().
>
> This wont be possible as we would extend Get with the generics
> <K,Collection<V>> as we want the method "get" to have a signature like
>
> Collection<V> get(Object key)
>
> Now, extending the Put interface with generics <K,V> does not pose that
> much of an issue except that the Map interface in Java 7 has a put
> signature as V put(K key, V value)  whereas the Collections 4 Put still has
> Object put(Key k, V value), but we can ignore this if we want.
>
> For the problem with Get, we can have a parallel MultiValuedGet and
> MultiValuedPut interfaces to honor the Get/Put split concepts. Although we
> don't really need the MultiValuedPut, we can have that for consistency.
>
> Let me know your thoughts on this.
>
> Regards
> Dipanjan
>
>
>
>
>
>
>
>
>
> On Wed, Feb 26, 2014 at 7:12 PM, Matt Benson <gudnabrsam@gmail.com> wrote:
>
>> Don't forget about the Get/Put/split map concepts from Collections 4. It
>> would seem you could implement those interfaces and provide that amount of
>> abstraction anyway.
>>
>> Matt
>> On Feb 26, 2014 3:26 AM, "Dipanjan Laha" <dipanjan21@gmail.com> wrote:
>>
>> > 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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