On Wed, 4 Nov 2020 23:03:02 GMT, Paŭlo Ebermann <github.com+645859+ep...@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/ImmutableCollections.java line 199: > >> 197: * safely reused as the List's internal storage, avoiding a >> defensive copy. Declared >> 198: * with Object... instead of E... as the parameter type so that >> varargs calls don't >> 199: * accidentally create an array of type other than Object[]. > > Why would that be a problem? If the resulting list is immutable, then the > actual array type doesn't really matter, right? It's an implementation invariant that the internal array be Object[]. Having it be something other than Object[] can lead to subtle bugs. See [JDK-6260652](https://bugs.openjdk.java.net/browse/JDK-6260652) for example. ------------- PR: https://git.openjdk.java.net/jdk/pull/1026
