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From "Adrian Nistor (JIRA)" <j...@apache.org>
Subject [jira] [Created] (COLLECTIONS-417) AbstractLinkedList.retainAll() is very slow
Date Fri, 29 Jun 2012 22:02:42 GMT
Adrian Nistor created COLLECTIONS-417:
-----------------------------------------

             Summary: AbstractLinkedList.retainAll() is very slow
                 Key: COLLECTIONS-417
                 URL: https://issues.apache.org/jira/browse/COLLECTIONS-417
             Project: Commons Collections
          Issue Type: Bug
    Affects Versions: 3.2.1
         Environment: java 1.6.0_24
Ubuntu 11.10
            Reporter: Adrian Nistor
         Attachments: Test.java, patch.diff

Hi,

I am encountering a performance problem in
AbstractLinkedList.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 158X speedup.

To run the test, just do:

$ java Test

The output for the un-patched version is:
Time is 5531

The output for the patched version is:
Time is 35

As the patch shows, the problem is that
"AbstractLinkedList.retainAll(Collection<?> coll)" performs
"coll.contains(it.next())" for each element in the AbstractLinkedList,
which can be very expensive if "coll.contains()" is expensive, e.g.,
when "coll" is a list.

The one-line patch I attached puts the elements of "coll" in a HashSet
(which has very fast "contains()"), if "coll" is not already a set:

"if (!(coll instanceof java.util.Set<?>)) coll = new java.util.HashSet<Object>(coll);"

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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