RFR: 8299444 java.util.Set.copyOf allocates needlessly for empty input collections

[Aleksey Shipilev]

On Wed, 4 Jan 2023 14:41:20 GMT, Viktor Klang <duke at openjdk.org> wrote:

> Currently Set.copyOf allocates both a HashSet and a new empty array when the input collection is empty.
> 
> This patch avoids allocating anything for the case where the parameter collection's isEmpty returns true.

Looks okay. This relies a bit on `isEmpty()` to be actually fast, but it seems unlikely it would be worse than doing the `toArray()`.

`Map.copyOf` has the same opportunity.

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Marked as reviewed by shade (Reviewer).

PR: https://git.openjdk.org/jdk/pull/11847