RFR: 6323374: (coll) Optimize Collections.unmodifiable* and synchronized* [v5]

[Stuart Marks]

On Fri, 26 Feb 2021 20:15:19 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:
> 
>   Test refactoring. Adding implNote to modified methods

The `@implNote` additions are good, and the test rewrite looks good too.

-------------

Marked as reviewed by smarks (Reviewer).

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