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https://issues.apache.org/jira/browse/COLLECTIONS-429?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Thomas Neidhart resolved COLLECTIONS-429.
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Resolution: Fixed
Fix Version/s: 4.0
Changed the method in r1469039 to a version similar in the patch.
The reason I created a new method instead of applying the patch is as follows:
* the outlined performance gain is based on an extreme and unlikely example,
in a typical use-case the object-creation may even outweigh the iteration.
* it would be unusual for Collections classes to duplicate it elements by
using any of the methods of the Collections interface. This could have negative
side-effects for unaware users, thus adding the method in CollectionUtils with
the described runtime/space trade-off.
Thanks for the patch anyway!
> Performance problem in MultiValueMap
> ------------------------------------
>
> Key: COLLECTIONS-429
> URL: https://issues.apache.org/jira/browse/COLLECTIONS-429
> Project: Commons Collections
> Issue Type: Improvement
> Affects Versions: 3.2.1
> Environment: java 1.6.0_24
> Ubuntu 11.10
> Reporter: Adrian Nistor
> Fix For: 4.0
>
> Attachments: patchFull_AbstractHashedMap.diff, patchFull.diff,
> patchFull_StaticBucketMap.diff, patchSmall_AbstractHashedMap.diff,
> patchSmall.diff, patchSmall_StaticBucketMap.diff,
> Test_AbstractHashedMap.java, TestDifferentParameter.java, Test.java,
> Test_StaticBucketMap.java
>
>
> Hi,
> I am encountering a performance problem in MultiValueMap. It appears
> in version 3.2.1 and also in revision 1366088. I attached a test that
> exposes this problem and a patch that fixes it. On my machine, for
> this test, the patch provides a 1158X speedup.
> To run the test, just do:
> $ java Test
> The output for the un-patched version is:
> Time is 44040
> The output for the patched version is:
> Time is 38
> The attached test shows that, for a "MultiValueMap multi" object, the
> following operation is very slow:
> multi.values().containsAll(toContain);
> "MultiValueMap.values()" returns a "MultiValueMap.Values" object,
> which inherits containsAll() from "java.util.AbstractCollection",
> which has quadratic complexity.
> I attached two patches. Both patches override containsAll() and
> implement a linear time algorithm. patchSmall.diff populates a
> HashSet eagerly, and patchFull.diff populates a HashSet lazily.
> patchFull.diff is faster than patchSmall.diff when containsAll()
> returns false after inspecting only a few elements, though in most
> cases they are equally fast. I am including patchSmall.diff just in
> case you prefer smaller code.
> Note that this problem is different from COLLECTIONS-416. As
> established in the COLLECTIONS-416 discussion, there the user was
> responsible for using the proper data structures as argument for the
> method.
> For "MultiValueMap.values()", the problem is not related to the
> collection passed as parameter. The problem will always manifest for
> this method, irrespective of the parameter type. I attached a test
> (TestDifferentParameter.java) that shows that, even with a HashSet
> parameter, the current problem still manifests (which did not happen
> for COLLECTIONS-416).
> This problem also exists for the two other "Values" classes
> (AbstractHashedMap.Values, StaticBucketMap.Values). I attached tests
> and patches for these classes as well. If you want me to file
> separate reports, just let me know.
> Is this truly a performance problem, or am I misunderstanding the
> intended behavior? If so, can you please confirm that the patches are
> correct?
> Thanks,
> Adrian
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