RFR: 8180352: Add Stream.toList() method [v3]

[Rémi Forax]

On Wed, 4 Nov 2020 09:21:12 GMT, Tagir F. Valeev <tvaleev at openjdk.org> wrote:

>> src/java.base/share/classes/java/util/stream/Stream.java line 1192:
>> 
>>> 1190:     @SuppressWarnings("unchecked")
>>> 1191:     default List<T> toList() {
>>> 1192:         return (List<T>) Collections.unmodifiableList(new ArrayList<>(Arrays.asList(this.toArray())));
>> 
>> Why can't we return `listFromTrustedArrayNullsAllowed` here as in `ReferencePipeline`?
>> Or at least, we should avoid unnecessary copying of arrays. See [how this is done](https://github.com/amaembo/streamex/blob/master/src/main/java/one/util/streamex/AbstractStreamEx.java#L1313) in StreamEx.
>
> `listFromTrustedArrayNullsAllowed` is clearly not an option, as it will produce shared-secret leaking (see [JDK-8254090](https://bugs.openjdk.java.net/browse/JDK-8254090) for a similar case). StreamEx solution is dirty as it relies on the implementation detail. I believe, OpenJDK team is not very interested in providing optimal implementations for third-party stream implementations, as third-party implementations will likely update by themselves when necessary. At least, my suggestion to make the default `mapMulti` implementation better was declined.

This implementation duplicates the array twice, once in this.toArray() and once in the constructor of ArrayList that calls toArray() on Collections.ArrayList.

I'm sure there is a better way

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PR: https://git.openjdk.java.net/jdk/pull/1026