Review for CR 6728865 : Improved heuristics for Collections.disjoint() [updated]

Ulf Zibis Ulf.Zibis at
Tue Jan 4 01:02:08 UTC 2011

Am 03.01.2011 21:16, schrieb Mike Duigou:
> On Dec 24 2010, at 17:32 , Ulf Zibis wrote:
>> Am 23.12.2010 23:59, schrieb Paul Benedict:
>>> Ulf, a previous email by Remi said only to invoke size() if the collection is a Set.
>> Thanks I missed that.
>> My guess was, that size() should be always faster than instantiating an iterator for the final for-loop, and then seeing, that there is no element.
> An earlier webrev had an isEmpty() check in the "c1 is a Set" half of the branch. This optimization would pay off in some cases but cost a small amount in others. Since I don't have any strong sense of whether including it would be of actual benefit or harm I opted to take it out in later revisions.
For the same reason you could set the empty-check at 1st place. This would result in the most simple 
(and fast) code (see my before post).
- in all implementations, I have seen, "isEmpty()" is equivalent to "size() == 0"
- "cost a small amount in others" applies rarely; the only implementation, I have seen, where 
"Set.size() can be expensive", is ConcurrentSkipListSet (are there others?).
- swapping c1 , c2 only happens in the very rare applying cases.

Anyway, isn't size() anyhow cheaper than superfluously looping contains() ? E.g. Given Set c1 with 
100 elements and Set c2 with 0 elements. Then we iterate and compare over 100 elements needlessly.


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