RFR: JDK-8025610 : (xs) Optional constructor and Optional.of should explicitly document NPE

[Mike Duigou]

Hello all;

As pointed out by Roger Riggs, Optional.of (and the private Optional(T) 
constructor) don't explicitly document throwing NPE in response to a null value 
despite describing value as "non-null". This changeset improves the 
documentation to add an @throws NPE tag to the javadoc.

http://cr.openjdk.java.net/~mduigou/JDK-8025610/0/webrev/

Thanks,

Mike