Anonymous value classes

[Brian Goetz]

There is no chance to get any calling-convention optimization here, 
since the concrete class name will not show up in any method descriptor 
(or preload attribute).  There is no chance to get any heap flattening 
here, since the concrete class name will not show up in any field 
descriptor or `newarray` operand.

So the main argument is "for completeness", which seems weak.

On 6/3/2022 12:15 PM, Dan Smith wrote:
> Our javac prototype has long included support for a 'value' keyword after 'new' to indicate that an anonymous class is a value class:
>
> Runnable r = new value Runnable() {
>      public void run() { x.foo(); }
> };
>
> Is this something we'd like to preserve as a language feature?
>
> Arguments for:
>
> - Allows the semantics of "I don't need identity" to be conveyed (often true for anonymous classes).
>
> - Gives the JVM more information for optimization. If we don't need a heap object, evaluating the expression may be a no-op.
>
> Arguments against:
>
> - Opens a Pandora's box of syntax: what other keywords can go there? 'identity'? 'primitive'? 'static'? 'record'?
>
> - Because there's no named type, there are significantly fewer opportunities for optimization—you're probably going to end up with a heap object anyway.
>
> - Value classes are primarily focused on simple data-carrying use cases, but any data being carried by an anonymous class is usually incidental. A new language feature would draw a lot of attention to this out-of-the-mainstream use case.
>
> - In the simplest cases, you can use a lambda instead, and there the API implementation has freedom to implement lambdas with value classes if it turns out to be useful.
>
> - The workaround—declare a local class instead—is reasonably straightforward for the scenarios where there's a real performance bottleneck that 'value' can help with.