RFR: 8305774: String.join(CharSequence, Iterable) can be optimized if Iterable is a Collection [v2]

[Chen Liang]

On Sun, 9 Apr 2023 02:28:37 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:
> 
>   Add benchmark

I recommended toArray for concurrent collections usually optimize that better than size. The goal is to defend against malicious actors that produce broken objects (like method parameters with void type), and tossing a problem back to the actor is a good enough solution. All these broken toArray will immediately backfire to the caller or execute normally without side effects, so I think toArray is safe.

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PR Comment: https://git.openjdk.org/jdk/pull/13383#issuecomment-1501400056