RFR: 8199318: add idempotent copy operation for Map.Entry

[Alan Bateman]

On Wed, 2 Jun 2021 00:39:25 GMT, Stuart Marks <smarks at openjdk.org> wrote:

> I'm fixing this along with a couple intertwined issues.
> 
> 1. Add Map.Entry::copyOf as an idempotent copy operation.
> 
> 2. Clarify that AbstractMap.SimpleImmutableEntry is itself unmodifiable (not really immutable) but that subclasses can be modifiable.
> 
> 3. Clarify some confusing, historical wording about Map.Entry instances being obtainable only via iteration of a Map's entry-set view. This was never really true, since anyone could implement the Map.Entry interface, and it certainly was not true since JDK 1.6 when SimpleEntry and SimpleImmutableEntry were added.

I read through the javadoc and API notes and it all looks reasonable.

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

Marked as reviewed by alanb (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/4295