RFR(s): JDK-8214687 Optimize Collections.nCopies().hashCode()

Tagir Valeev amaembo at gmail.com
Tue Dec 4 05:22:46 UTC 2018


Thank you for your comments!

Yes, deserialization will be broken if we assume that size is never 0.
Also we'll introduce referential identity Collections.nCopies(0, x) ==
Collections.nCopies(0, y) which might introduce slight semantics
change in existing programs. Once I suggested to wire Arrays.asList()
(with no args) to Collections.emptyList(), but it was rejected for the
same reason: no need to introduce a risk of possible semantics change.

I updated webrev with equals implementation and test:
Comparing two CopiesList is much faster now indeed. Also we can spare
an iterator in the common case and hoist the null-check out of the
loop. Not sure whether we can rely that JIT will always do this for
us, but if you think that it's unnecessary, I can merge the loops
back. Note that now copiesList.equals(arrayList) could be faster than
arrayList.equals(copiesList). I don't think it's an issue. On the
other hand we could keep simpler and delegate to super-implementation
if the other object is not a CopiesList (like it's implemented in
java.util.RegularEnumSet::equals for example). What do you think?

With best regards,
Tagir Valeev.
On Tue, Dec 4, 2018 at 10:56 AM Stuart Marks <stuart.marks at oracle.com> wrote:
> >> I believe it makes sense to override CopiesList.equals()
> > Also: contains(), iterator(), listIterator()
> equals(): sure
> contains() is already overridden. Not sure there's much benefit to overriding
> the iterators.
> s'marks

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