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From "Adrian Nistor (JIRA)" <j...@apache.org>
Subject [jira] [Created] (COLLECTIONS-418) ListUtils.retainAll() is very slow
Date Fri, 29 Jun 2012 22:16:44 GMT
Adrian Nistor created COLLECTIONS-418:
-----------------------------------------

             Summary: 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


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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