RFR 8233187: Return Empty List if 0 Copies Requested in Collections nCopies

[Stuart Marks]

I just added this comment to the bug report, but since there's a review thread,
I'll post it here too for discussion:

> I don't necessarily think that returning the instance from
> Collections.emptyList() is a good idea. It changes the serialized format, and
> it may also introduce some subtle behavioral changes. The nCopies() method
> could instead return a cached instance of a zero-length CopiesList instance.
> I question whether this is worthwhile. Is there an actual case where somebody
> is creating large numbers of empty lists, or is this just some random idea
> somebody thought up? If the latter, then it's usually not worth pursuing such
> changes.

There's no justification for this stated in the bug report, e.g., "we ran into a 
case where millions of redundant empty lists were being created...." It smells 
to me like this is one of those "optimizations" where somebody looked at the 
code and decided it would be a good idea.

s'marks




On 11/14/19 4:34 PM, Martin Buchholz wrote:
> Hi Kiran, pleased to meet you
> 
> Code like this should be written so that the common case has only one boolean
> test.
> 
> On Thu, Nov 14, 2019 at 1:50 AM Kiran Ravikumar < 
> kiran.sidhartha.ravikumar at oracle.com> wrote:
> 
>> Hi Guys,
>> 
>> 
>> Please review the following optimization to nCopies method to return empty
>> list and to avoid instantiating an empty CopiesList object.
>> 
>> 
>> JBS link : https://bugs.openjdk.java.net/browse/JDK-8233187
>> 
>> 
>> Patch:
>> 
>> diff -r f279d8a9bb78
>> src/java.base/share/classes/java/util/Collections.java ---
>> a/src/java.base/share/classes/java/util/Collections.java Wed Nov 13 
>> 11:27:50 2019 +0000 +++
>> b/src/java.base/share/classes/java/util/Collections.java Thu Nov 14 
>> 09:37:56 2019 +0000 @@ -5105,6 +5105,8 @@ public static <T> List<T>
>> nCopies(int n, T o) { if (n < 0) throw new IllegalArgumentException("List
>> length = " + n); +        if (n == 0) +            return
>> Collections.emptyList(); return new CopiesList<>(n, o); }
>> 
>> 
>> Thanks,
>> 
>> Kiran
>> 
>>