A disclaimer or two for Optional

[Brian Goetz]

This is a nice goal, except that it's patently unreasonable :). It is not possible for the compiler to understand and enforce every library-based invariant.  



Sent from my iPad

On Nov 30, 2013, at 11:43 AM, Joe Bowbeer <joe.bowbeer at gmail.com> wrote:

> I want javac to warn about any code that may have unpredictable results. This is even more important if the unpredictable results may not manifest immediately.
> 
> On Saturday, November 30, 2013, Brian Goetz wrote:
>> No javac interaction for now. 
>> 
>> Sent from my iPhone
>> 
>> On Nov 27, 2013, at 7:43 PM, Joe Bowbeer <joe.bowbeer at gmail.com> wrote:
>> 
>>> I approve. It seems definitive.
>>> 
>>> It is also scary. Will javac emit these warnings?
>>> 
>>> On Nov 26, 2013 1:36 PM, "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>