RFR: 8305774: String.join(CharSequence, Iterable) can be optimized if Iterable is a Collection [v3]

[jmehrens]

On Mon, 10 Apr 2023 05:46:23 GMT, Tingjun Yuan <duke at openjdk.org> wrote:

>> In the current implementation of `String.join(CharSequence, Iterable)`, the temp array `elems` is always initialized with a length of 8. It will cause many array recreations when the `Iterable` contains more than 8 elements. Furthermore, it's very common that an `Iterable` is also a `Collection`. So if the `Iterable` is an instance of `Collection`, the initial length of the array can be `((Collection<?>)elements).size()`. It will not change the current behavior even if the `Collection` is modified asynchronously.
>> 
>> I don't know whether this change requires a CSR request.
>
> Tingjun Yuan has updated the pull request incrementally with one additional commit since the last revision:
> 
>   use spliterator().estimateSize()

src/java.base/share/classes/java/lang/String.java line 3466:

> 3464:         }
> 3465:         int size = 0;
> 3466:         for (CharSequence cs: elements) {

I would think you have to locally store the result of `elements.spliterator()` and then use Spliterators::iterator to adapt it back to an iterator.  This should correctly handle [early-binding](https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/util/Spliterator.html#binding) spliterators.

I think in the loop the code should use ArraysSupport.newLength.

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PR Review Comment: https://git.openjdk.org/jdk/pull/13383#discussion_r1161712521