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Posted to user@commons.apache.org by Ryan Bennetts <mo...@gmail.com> on 2014/03/27 00:59:32 UTC
[collections] MultiValueMap with a custom value collection type
Hello all
I am having trouble understanding how to use a custom value collection type
with a MultiValueMap. I want to use the new generics-supported version, but
can't see how to do this without excessive casting, which seems to defeat a
lot of the purpose of using generics in the first place. I have posted this
issue on stackoverflow <http://stackoverflow.com/questions/22625575> but
thought I would ask here too.
I would like a MultiValueMap<http://commons.apache.org/proper/commons-collections/javadocs/api-release/org/apache/commons/collections4/map/MultiValueMap.html>which
takes a String as a key, and a collection of Strings as the value.
But:
1. The keys should retain insertion ordering (so I create the
multi-valued map by decorating a
LinkedHashMap<http://docs.oracle.com/javase/6/docs/api/java/util/LinkedHashMap.html>
)
2. The values should be unique for each key and retain insertion
ordering (so I want the values Collection type to be a
LinkedHashSet<http://docs.oracle.com/javase/6/docs/api/java/util/LinkedHashSet.html>
).
The closest I can get is:
MultiValueMap<String, String> orderedMap = MultiValueMap.multiValueMap(
new LinkedHashMap<String, Collection<String>>(),
LinkedHashSet.class);
But that produces this error:
The method multiValueMap(Map<K,? super C>, Class<C>) in the type
MultiValueMap is not applicable for the arguments
(LinkedHashMap<String,Collection<String>>,
Class<LinkedHashSet>)
So now I am in generics hell. Any suggestions would be most welcome. One
suggestion I have received is to use:
@SuppressWarnings("unchecked")MultiValueMap<String, String> orderedMap
= MultiValueMap.multiValueMap(
new LinkedHashMap<String, Collection<String>>(),
(Class<LinkedHashSet<String>>)(Class<?>)LinkedHashSet.class);
This works but do I really have to do two casts? Is this really what the
api designers intended?
Prior to version 4.0, I accomplished that with the following:
MultiValueMap orderedMap = MultiValueMap.decorate(
new LinkedHashMap<>(),
LinkedHashSet.class);
Which is certainly simpler, but not type safe. But that requires casting
when I call put() and get() and so I'd like to be able to use the new
generic version provided by 4.0.
Thanks,
-- Ryan
Re: [collections] MultiValueMap with a custom value collection type
Posted by Ryan Bennetts <mo...@gmail.com>.
Hi Thomas
Thanks very much for your reply. I'll look forward to version 4.1, but in
the meantime I'll go with your suggested code.
Thanks again,
-- Ryan
> Hello all
> >
> > I am having trouble understanding how to use a custom value collection
> type
> > with a MultiValueMap...
>
> Hi Ryan,
>
> we know that the MultiValueMap is flawed for several reasons, but we are
> working on adding a revamped variant in 4.1, see the issue:
>
> https://issues.apache.org/jira/browse/COLLECTIONS-508
>
> The best thing you can do right now is to instantiate it in the
> pre-generics way:
>
> @SuppressWarnings({ "rawtypes", "unchecked" })
> MultiValueMap<String, String> orderedMap
> = MapUtils.multiValueMap(new LinkedHashMap(), LinkedHashSet.class);
>
> You actually need type-safety for orderedMap, not for the argument to
> multiValueMap. The need to suppress warnings is annoying, but with 4.1
> this should also go away.
>
> Thomas
>
Re: [collections] MultiValueMap with a custom value collection type
Posted by Thomas Neidhart <th...@gmail.com>.
On 03/27/2014 12:59 AM, Ryan Bennetts wrote:
> Hello all
>
> I am having trouble understanding how to use a custom value collection type
> with a MultiValueMap. I want to use the new generics-supported version, but
> can't see how to do this without excessive casting, which seems to defeat a
> lot of the purpose of using generics in the first place. I have posted this
> issue on stackoverflow <http://stackoverflow.com/questions/22625575> but
> thought I would ask here too.
>
> I would like a MultiValueMap<http://commons.apache.org/proper/commons-collections/javadocs/api-release/org/apache/commons/collections4/map/MultiValueMap.html>which
> takes a String as a key, and a collection of Strings as the value.
> But:
>
> 1. The keys should retain insertion ordering (so I create the
> multi-valued map by decorating a
> LinkedHashMap<http://docs.oracle.com/javase/6/docs/api/java/util/LinkedHashMap.html>
> )
> 2. The values should be unique for each key and retain insertion
> ordering (so I want the values Collection type to be a
> LinkedHashSet<http://docs.oracle.com/javase/6/docs/api/java/util/LinkedHashSet.html>
> ).
>
> The closest I can get is:
>
> MultiValueMap<String, String> orderedMap = MultiValueMap.multiValueMap(
> new LinkedHashMap<String, Collection<String>>(),
> LinkedHashSet.class);
>
> But that produces this error:
>
> The method multiValueMap(Map<K,? super C>, Class<C>) in the type
> MultiValueMap is not applicable for the arguments
> (LinkedHashMap<String,Collection<String>>,
> Class<LinkedHashSet>)
>
> So now I am in generics hell. Any suggestions would be most welcome. One
> suggestion I have received is to use:
>
> @SuppressWarnings("unchecked")MultiValueMap<String, String> orderedMap
> = MultiValueMap.multiValueMap(
> new LinkedHashMap<String, Collection<String>>(),
> (Class<LinkedHashSet<String>>)(Class<?>)LinkedHashSet.class);
>
> This works but do I really have to do two casts? Is this really what the
> api designers intended?
>
> Prior to version 4.0, I accomplished that with the following:
>
> MultiValueMap orderedMap = MultiValueMap.decorate(
> new LinkedHashMap<>(),
> LinkedHashSet.class);
>
> Which is certainly simpler, but not type safe. But that requires casting
> when I call put() and get() and so I'd like to be able to use the new
> generic version provided by 4.0.
Hi Ryan,
we know that the MultiValueMap is flawed for several reasons, but we are
working on adding a revamped variant in 4.1, see the issue:
https://issues.apache.org/jira/browse/COLLECTIONS-508
The best thing you can do right now is to instantiate it in the
pre-generics way:
@SuppressWarnings({ "rawtypes", "unchecked" })
MultiValueMap<String, String> orderedMap
= MapUtils.multiValueMap(new LinkedHashMap(), LinkedHashSet.class);
You actually need type-safety for orderedMap, not for the argument to
multiValueMap. The need to suppress warnings is annoying, but with 4.1
this should also go away.
Thomas
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