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https://issues.apache.org/jira/browse/COLLECTIONS-418?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Thomas Neidhart resolved COLLECTIONS-418.
-----------------------------------------
Resolution: Won't Fix
Added to the javadoc a clarification on the runtime complexity of the method.
Users shall use a data structure for the elements to be retained which supports
a fast implementation of contains.
> ListUtils.retainAll() is very slow
> ----------------------------------
>
> Key: COLLECTIONS-418
> URL: https://issues.apache.org/jira/browse/COLLECTIONS-418
> Project: Commons Collections
> Issue Type: Bug
> Affects Versions: 3.2.1
> Environment: java 1.6.0_24
> Ubuntu 11.10
> Reporter: Adrian Nistor
> Attachments: patch.diff, Test.java
>
>
> Hi,
> I am encountering a performance problem in ListUtils.retainAll(). It
> appears in version 3.2.1 and also in revision 1355448. I attached a
> test that exposes this problem and a one-line patch that fixes it. On
> my machine, for this test, the patch provides a 238X speedup.
> To run the test, just do:
> $ java Test
> The output for the un-patched version is:
> Time is 5485
> The output for the patched version is:
> Time is 23
> As the patch shows, the problem is that
> "ListUtils.retainAll(Collection<E> collection, Collection<?> retain)"
> performs "retain.contains(obj)" for each element in "collection",
> which can be very expensive if "retain.contains(obj)" is expensive,
> e.g., when "retain" is a list.
> The one-line patch I attached puts the elements of "retain" in a
> HashSet (which has very fast "contains()"), if "retain" is not already
> a set:
> "if (!(retain instanceof java.util.Set<?>)) retain = new
> HashSet<Object>(retain);"
> Is this a bug, or am I misunderstanding the intended behavior? If so,
> can you please confirm that the patch is correct?
> Thanks,
> Adrian
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