RFR 8022797 Clarify spliterator characteristics for collections containing no elements

[Paul Sandoz]

Hi,

Here is another go at characteristics reported for spliterators of empty collections:

diff -r f8d92ce66886 src/share/classes/java/util/Collection.java
--- a/src/share/classes/java/util/Collection.java	Mon Aug 12 13:59:01 2013 +0200
+++ b/src/share/classes/java/util/Collection.java	Mon Aug 12 16:12:10 2013 +0200
@@ -504,7 +504,10 @@
      *
      * <p>The returned {@code Spliterator} must report the characteristic
      * {@link Spliterator#SIZED}; implementations should document any additional
-     * characteristic values reported by the returned Spliterator.
+     * characteristic values reported by the returned spliterator.  If
+     * this collection contains no elements then the returned spliterator is
+     * only required to report {@link Spliterator#SIZED} and is not required to
+     * report additional characteristics (if any).
      *
      * <p>The default implementation should be overridden by subclasses that
      * can return a more efficient spliterator.  In order to
@@ -535,6 +538,13 @@
      * The returned {@code Spliterator} additionally reports
      * {@link Spliterator#SUBSIZED}.
      *
+     * <p>If a spliterator covers no elements then the reporting of additional
+     * characteristics, beyond that of {@code SIZED} and {@code SUBSIZED}, does
+     * not aid clients to control specialize or simplify computation.  However,
+     * this does enable shared use of an empty spliterator (see
+     * {@link Spliterators#emptySpliterator()} for empty collections, and
+     * enables clients to determine if a spliterator covers no elements.
+     *
      * @return a {@code Spliterator} over the elements in this collection
      * @since 1.8
      */

It might be preferable to also document this on List/Set/SortedSet since the documentation is overridden and i don't think we have many precedents for overriding what the sub-types define (are there any?).

--

I logged issue 8022805 for Stream.concat(s, Stream.emptyStream()).

Paul.