RFR: 6323374: (coll) Optimize Collections.unmodifiable* and synchronized* [v4]

[Stuart Marks]

On Fri, 19 Feb 2021 01:52:51 GMT, liach <github.com+7806504+liach at openjdk.org> wrote:

>> Maybe it is not correct for UnmodifiableEntrySet::contains to short circuit?  What if the implementation was changed to:
>> 
>> `public boolean contains(Object o) {
>>             if (!(o instanceof Map.Entry))
>>                 return c.contains(o); //false, NPE, or CCE
>>             return c.contains(
>>                 new UnmodifiableEntry<>((Map.Entry<?,?>) o));
>> }`
>
> This however changes the behavior of unmodifiable maps compared to before; i.e. before for other entry sets, they could not throw exception if the object passed was not map entry; now they can.

Yes, it seems reasonable to decouple the `ImmutableCollections` issues from this change.

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PR: https://git.openjdk.java.net/jdk/pull/2596