RFR: 8180352: Add Stream.toList() method

[Stuart Marks]

On Tue, 3 Nov 2020 11:05:21 GMT, Stephen Colebourne <scolebourne 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 1168:
> 
>> 1166:      * Accumulates the elements of this stream into a {@code List}. The elements in
>> 1167:      * the list will be in this stream's encounter order, if one exists. There are no
>> 1168:      * guarantees on the implementation type, mutability, serializability, or
> 
> It would be useful for callers to feel more confident that they will get an immutable instance. In java.time.* we have wording like "This interface places no restrictions on the mutability of implementations, however immutability is strongly recommended." Could something like that work here, emphasising that everyone implementing this method should seek to return an immutable list?

Yes, good point, the "no guarantee of mutability" clashes with the later statement about the possibility of the returned instance being value-based, which strongly implies immutability. I'll work on tuning this up to be a stronger statement on immutability, while retaining "no-guarantee" for implementation type, serializability, etc. I think we do want to preserve future implementation flexibility in those areas.

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