RFR: 6323374: (coll) Optimize Collections.unmodifiable* and synchronized* [v3]

[jmehrens]

On Wed, 17 Feb 2021 19:12:19 GMT, Ian Graves <igraves at openjdk.org> wrote:

>> This raises some interesting issues and makes me wonder if we should allow a single-wrap of the `ImmutableCollections` classes for now to make this less onerous.
>
> Yes -- I think in response to this it makes more sense to pull the `ImmutableCollections` classes out for now and only focus on the wrapping of the classes within `Collections` so we aren't blocked by studying and rectifying these inconsistencies.

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));
}`

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