A disclaimer or two for Optional

[Ali Ebrahimi]

Hi, Not all devs read javadocs, so I think in such cases javac can inform
(warn) them. So I think annotating such types would be usefull (@ValueType
or even value modifier in class file) similar to @Override.


On Wed, Nov 27, 2013 at 1:05 AM, Brian Goetz <brian.goetz at oracle.com> 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>
>
>