On Wed, 4 Nov 2020 08:50:49 GMT, Zheka Kozlov <github.com+761899+orio...@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. ------------- PR: https://git.openjdk.java.net/jdk/pull/1026
