[PATCH] Minor optimisation for Set.copyOf(Collection)

[Tagir Valeev]

Hello!

What if it's TreeSet with custom comparator which may differentiate
elements equal by .equals()? Or, even simpler,
Collections.newSetFromMap(new IdentityHashMap<>())? In both cases
Set.of(array) would throw.

With best regards,
Tagir Valeev.

On Fri, Jun 22, 2018 at 4:43 PM Andrej Golovnin <andrej.golovnin at gmail.com>
wrote:

> > Hi all,
> >
> > the current implementation of Set.copyOf(Collection<? extends E> coll)
> > creates an intermediate HashSet object to remove duplicates even when
> > coll is already a Set. The attached patch adds an additional check
> > whether coll is already a Set and if yes, then calls #toArray()
> > directly on coll and passes the result to Set.of().
>
> It looks like my patch was stripped. Now inlined:
>
> diff --git a/src/java.base/share/classes/java/util/Set.java
> b/src/java.base/share/classes/java/util/Set.java
> --- a/src/java.base/share/classes/java/util/Set.java
> +++ b/src/java.base/share/classes/java/util/Set.java
> @@ -723,6 +723,8 @@
>      static <E> Set<E> copyOf(Collection<? extends E> coll) {
>          if (coll instanceof ImmutableCollections.AbstractImmutableSet) {
>              return (Set<E>)coll;
> +        } else if (coll instanceof Set<?>) {
> +            return (Set<E>)Set.of(coll.toArray());
>          } else {
>              return (Set<E>)Set.of(new HashSet<>(coll).toArray());
>          }
>