[PATCH] java.util.Collections enhancement

[David Holmes]

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)