RFR(m): 8177290 add copy factory methods for unmodifiable List, Set, Map
stuart.marks at oracle.com
Wed Nov 1 19:25:21 UTC 2017
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
I think the only safe way to avoid the extra copy is to create a private
interface that can be used by JDK implementations.
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