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From "Adrian Nistor (JIRA)" <j...@apache.org>
Subject [jira] [Created] (COLLECTIONS-419) Performance problem in AbstractDualBidiMap
Date Fri, 29 Jun 2012 22:59:42 GMT
Adrian Nistor created COLLECTIONS-419:
-----------------------------------------

             Summary: Performance problem in AbstractDualBidiMap
                 Key: COLLECTIONS-419
                 URL: https://issues.apache.org/jira/browse/COLLECTIONS-419
             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 AbstractDualBidiMap.  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 130X speedup.

To run the test, just do:

$ java Test

The output for the un-patched version is:
Time is 5460

The output for the patched version is:
Time is 42

The attached test shows that, for a "DualHashBidiMap bidi" object, the
following operation is very slow:

bidi.keySet().retainAll(toRetain)

DualHashBidiMap.keySet() returns a "DualHashBidiMap.KeySet" object,
which inherits "retainAll(Collection<?> coll)" from
"AbstractDualBidiMap.View". Similarly,

bidi.values().retainAll(toRetain)
bidi.entrySet().retainAll(toRetain)

are also slow.  This happens for both DualHashBidiMap and
DualTreeBidiMap, which extend AbstractDualBidiMap.

As the patch shows, the problem is that
"AbstractDualBidiMap.View.retainAll(Collection<?> coll)" performs
"coll.contains(it.next())" for each element in the View.
"coll.contains(it.next())" can be very slow, e.g., if "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 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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