> Am 01.11.2017 um 20:25 schrieb Stuart Marks <stuart.ma...@oracle.com>: > > On 10/31/17 5:52 PM, Bernd Eckenfels wrote: >> Having a List.of(List) copy constructor would save an additional array copy >> in the collector Which uses (List<T>)List.of(list.toArray()) > > The quickest way to get all the elements from the source collection is to > call toArray() on it. Some copy constructors (like ArrayList's) simply use > the returned array for internal storage. This saves a copy, but it relies on > the source collection's toArray() implementation to be correct. In > particular, the returned array must be freshly created, and that array must > be of type Object[]. If either of these is violated, it will break the > ArrayList. > > The "immutable" collections behind List.of/copyOf/etc. are fastidious about > allocating their internal arrays in order to ensure that they are referenced > only from within the newly created collection. This requires making an > „extra" copy of the array returned by toArray(). > > An alternative is for the new collection to preallocate its internal array > using the source's size(), and then to copy the elements out. But the source’s > size might change during the copy (e.g., if it’s a concurrent collection) so > this complicates things.
I think the array „overhead“ would be only for the cases of zero, one and two value implementations. That seems to me not worth of optimizing… > I think the only safe way to avoid the extra copy is to create a private > interface that can be used by JDK implementations. > > s'marks -Patrick
