RFR: 8180352: Add Stream.toList() method

[Tagir F.Valeev]

On Wed, 4 Nov 2020 08:50:49 GMT, Zheka Kozlov <github.com+761899+orionll at openjdk.org> wrote:

>> This change introduces a new terminal operation on Stream. This looks like a convenience method for Stream.collect(Collectors.toList()) or Stream.collect(Collectors.toUnmodifiableList()), but it's not. Having this method directly on Stream enables it to do what can't easily by done by a Collector. In particular, it allows the stream to deposit results directly into a destination array (even in parallel) and have this array be wrapped in an unmodifiable List without copying.
>> 
>> In the past we've kept most things from the Collections Framework as implementations of Collector, not directly on Stream, whereas only fundamental things (like toArray) appear directly on Stream. This is true of most Collections, but it does seem that List is special. It can be a thin wrapper around an array; it can handle generics better than arrays; and unlike an array, it can be made unmodifiable (shallowly immutable); and it can be value-based. See John Rose's comments in the bug report:
>> 
>> https://bugs.openjdk.java.net/browse/JDK-8180352?focusedCommentId=14133065&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-14133065
>> 
>> This operation is null-tolerant, which matches the rest of Streams. This isn't specified, though; a general statement about null handling in Streams is probably warranted at some point.
>> 
>> Finally, this method is indeed quite convenient (if the caller can deal with what this operation returns), as collecting into a List is the most common stream terminal operation.
>
> 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.

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