Add a Map<K,V> Maps.combinedValueIndex(Iterable<I>, Function<I,K>, Combiner<I,V>) for combination of multiple inputs per key into one value

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I would like something to help with transforming Iterable etc to a map 
where multiple inputs for a key can be combined into a single value.

It's like uniqueIndex, but instead of throwing an exception on the second 
time a key is calculated from the input, it is then sent to the Combiner to 
be resolved:

V Combiner.combine(I newInput, V existingValue);

(this would be called the first time too, where existingValue is null).

The separation of Input and Value may or may not be necessary. I think it's 
probably valid to only combine V values, and then transform the resultant 
Map.

Original issue reported on code.google.com by stephen....@gmail.com on 24 Feb 2010 at 2:15

[GoogleCodeExporter]

This sort of thing is most generally useful for mapping by "latest/earliest" 
value per 
calculated key, but affords itself to other uses (e.g. implementation of 
first/last in 
wins and other filterable criteria)

Original comment by stephen....@gmail.com on 24 Feb 2010 at 2:21

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Also, just to make sure we're not just chasing after a sorted-set-multimap 
first idea, 
it's good for set functions (sums, averages etc)

Original comment by stephen....@gmail.com on 24 Feb 2010 at 2:57

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Original comment by fry@google.com on 28 Jan 2011 at 3:59

• Changed state: Accepted

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Original comment by kevinb@google.com on 13 Jul 2011 at 6:18

• Changed state: Triaged

[GoogleCodeExporter]

We are not in favor of taking Guava further in this functional direction and 
are holding off on adding any new functional types like Consumer,Reducer,etc.  
You might want to use Functional Java (but that's not a recommendation).

The world will look much different in Java 8 and will be much friendlier to 
this style of programming.

Original comment by kevinb@google.com on 28 Jul 2011 at 8:29

• Changed state: WorkingAsIntended
• Added labels: Type-Enhancement

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This issue has been migrated to GitHub.

It can be found at https://github.com/google/guava/issues/<id>

Original comment by cgdecker@google.com on 1 Nov 2014 at 4:15

• Added labels: MigratedToGitHub

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Original comment by cgdecker@google.com on 3 Nov 2014 at 9:10

• Added labels: Restrict-AddIssueComment-Commit