RFR: 6323374: (coll) Optimize Collections.unmodifiable* and synchronized* [v8]

[Joe Darcy]

On Thu, 4 Mar 2021 22:35:09 GMT, Ian Graves <igraves at openjdk.org> wrote:

>> Modify the `unmodifiable*` methods in `java.util.Collections` to be idempotent. That is, when given an immutable collection from `java.util.ImmutableCollections` or `java.util.Collections`, these methods will return the reference instead of creating a new immutable collection that wraps the existing one.
>
> Ian Graves has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Commenting intent to not check for subclasses

Marked as reviewed by darcy (Reviewer).

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