RFR: 7057785 : (xs) Add note to hashCode() that support for self referential is optional

[Mike Duigou]

Hello all;

Fairly frequently it is reported that various Collection/Map implementations of hashCode() fail when the instance directly or indirectly contains itself. For a variety of reasons, mostly performance and resource related, most implementations choose not to support calculation of hash codes for self-referential collections. This is not likely to change. So to reduce confusion and "bug" reports I am proposing a non-normative @apiNote be added to Collection and HashMap. The text of the proposed note is:

> Support for calculation of hash code by self referential {Collection|Map}s (they either directly or indirectly contain themselves) is optional. Few Collection implementations support calculation of hash code for self referential instances.


http://cr.openjdk.java.net/~mduigou/JDK-7057785/0/webrev/

Cheers,

Mike