[COLLECTIONS-415] AbstractLinkedList.removeAll() is very slow

• Type: Bug
• Status: Closed
• Priority: Major
• 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 21:02
• Updated: 09/Nov/14 14:31
• Resolved: 20/Sep/12 18:08

Description

Hi,

I am encountering a performance problem in
AbstractLinkedList.removeAll(). 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 226X speedup.

To run the test, just do:

$ java Test

The output for the un-patched version is:
Time is 5655

The output for the patched version is:
Time is 25

As the patch shows, the problem is that
"AbstractLinkedList.removeAll(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