Also note that this changes visible behavior. E.g. somebody might synchronize on the array object returned by toArray call, so this change might cause unwanted lock sharing.
Once I suggested similar improvement: to return EMPTY_LIST from Arrays.asList() when supplied array has zero length. It was declined with the same reason [1] With best regards, Tagir Valeev. [1] http://mail.openjdk.java.net/pipermail/core-libs-dev/2015-September/035197.html On Tue, Sep 12, 2017 at 4:27 AM, David Holmes <david.hol...@oracle.com> wrote: > Moving to core-libs-dev ... > > On 12/09/2017 5:40 AM, Сергей Цыпанов wrote: >> >> Hi, >> >> looking into the code of java.util.Collections I've found out that >> EmptyList#toArray() and EmptySet#toArray() both instantiate new instance of >> Object[] at each call. As far as Java array is immutable it's possible to >> cache empty Object array and return this cached instance. > > > There is a slight issue with the specification of toArray here: > > "The returned array will be "safe" in that no references to it are > maintained by this list. (In other words, this method must allocate a new > array even if this list is backed by an array). " > > Of course for a zero-sized array it is perfectly safe to return a cached > array, but the spec currently requires that a new array be returned. > > Cheers, > David > ----- > >> Patch.txt is attached to the body of this mail. >> >> Thanks, >> - Sergei Tsypanov >> >> patch.txt >> >> diff --git a/src/java.base/share/classes/java/util/Collections.java >> b/src/java.base/share/classes/java/util/Collections.java >> --- a/src/java.base/share/classes/java/util/Collections.java >> +++ b/src/java.base/share/classes/java/util/Collections.java >> @@ -108,6 +108,11 @@ >> private static final int INDEXOFSUBLIST_THRESHOLD = 35; >> /** >> + * Cached empty array >> + */ >> + private static final Object[] EMPTY_ARRAY = {}; >> + >> + /** >> * Sorts the specified list into ascending order, according to the >> * {@linkplain Comparable natural ordering} of its elements. >> * All elements in the list must implement the {@link Comparable} >> @@ -4329,7 +4334,7 @@ >> public boolean contains(Object obj) {return false;} >> public boolean containsAll(Collection<?> c) { return c.isEmpty(); >> } >> - public Object[] toArray() { return new Object[0]; } >> + public Object[] toArray() { return EMPTY_ARRAY; } >> public <T> T[] toArray(T[] a) { >> if (a.length > 0) >> @@ -4458,7 +4463,7 @@ >> public boolean contains(Object obj) {return false;} >> public boolean containsAll(Collection<?> c) { return c.isEmpty(); >> } >> - public Object[] toArray() { return new Object[0]; } >> + public Object[] toArray() { return EMPTY_ARRAY; } >> public <T> T[] toArray(T[] a) { >> if (a.length > 0) > >
