A disclaimer or two for Optional

[Brian Goetz]

Draft of final proposed spec at:
   http://cr.openjdk.java.net/~mduigou/JDK-8028816/0/webrev/

On 11/26/2013 4:35 PM, Brian Goetz wrote:
> OK, after several iterations with Doug and JohnR, I think we have a winner.
>
> Then Optional would have a disclaimer:
>
>   * <p>This is a <a
> href="../package-summary.html#ValueBased">value-based</a>
>   * class; use of identity-sensitive operations on instances of {@code
> Optional}
>   * may have unpredictable effects and should be avoided.
>   *
>
> pointing to (which lives in some suitable place, probably a .html file
> java/lang/doc-files):
>
>
> <h2><a name="ValueBased">Value-based classes</a></h2>
>
> Some classes, such as <code>java.util.Optional</code> and
> <code>java.time.LocalDateTime</code>, are <em>value-based</em>.
> Instances of a
> value-based class:
> <ul>
>      <li>are immutable (though may contain references to mutable
> objects);</li>
>      <li>have value-based implementations of <code>equals</code>,
>          <code>hashCode</code>, and <code>toString</code>, which are
> computed
>          solely from the instance's state and not on its identity or the
> state
>          of any other object;</li>
>      <li>make no use of identity-sensitive operations such as reference
>          equality between instances, identity hash code of instances, or
>          synchronization on an instances's intrinsic lock;</li>
>      <li>are considered equal solely based on
> <code>Object.equals()</code>, not
>          based on reference equality (<code>==</code>);</li>
>      <li>are not instantiated through accessible constructors, but instead
>          through factory methods which make no committment as to the
> identity
>          of returned instances;</li>
>      <li>are <em>freely substitutable</em> when equal, meaning that
> interchanging
>          any two instances <code>x</code> and <code>y</code> that are equal
>          according to <code>Object.equals()</code> in any computation or
> method
>          invocation should produce no visible change in behavior.
>          </li>
> </ul>
>
> <p>A program may produce unpredictable results if it attempts to
> distinguish two
> references to equal values of a value-based class, whether directly via
> reference
> equality or indirectly via an appeal to synchronization, identity hashing,
> serialization, or any other identity-sensitive mechanism.  Use of
> identity-sensitive operations on instances of value-based classes may have
> unpredictable effects and should be avoided.</p>
>