[COLLECTIONS-419] Performance problem in AbstractDualBidiMap

• Type: Improvement
• Status: Closed
• Priority: Minor
• Resolution: Fixed
• Affects Version/s: 3.2.1
• Fix Version/s: 4.0-alpha1, 4.0

Environment

java 1.6.0_24
Ubuntu 11.10

• Assignee: Unassigned
• Reporter: Adrian Nistor
• Created: 29/Jun/12 22:58
• Updated: 09/Nov/14 14:31
• Resolved: 20/Apr/13 15:06

Description

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