Anonymous value classes

[Dan Smith]

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.