How can you extend Java to introduce passing by reference?

Question

Java is pass-by-value. How could you modify the language to introduce passing by reference (or some equivalent behavior)?

Take for example something like

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Answer 1

To answer your question:

Where can this fail?

1. Final variables and enum constants
2. 'Special' references such as this
3. References that are returned from method calls, or constructed inline using new
4. Literals (Strings, integers, etc.)

...and possibly others. Basically, your ref keyword must only be usable if the parameter source is a non-final field or local variable. Any other source should generate a compilation error when used with ref.

An example of (1):

final String s = "final";
passByReference(ref s);  // Should not be possible

An example of (2):

passByReference(ref this);  // Definitely impossible

An example of (3):

passByReference(ref toString());  // Definitely impossible
passByReference(ref new String("foo"));  // Definitely impossible

An example of (4):

passByReference(ref "literal");  // Definitely impossible

And then there are assignment expressions, which seem to me like something of a judgement call:

String s;
passByReference(ref (s="initial"));  // Possible, but does it make sense?

It's also a little strange that your syntax requires the ref keyword for both the method definition and the method invocation. I think the method definition would be sufficient.