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

Answering you question about how to extend the language my pick would be: - Using various holders technics as several other answers describe - Use annotations to attach metadata regarding which arguments should be passed by reference and then start juggling with a byte code manipulation library, like cglib in order to fulfil your ideas in byte code itself.

Though this whole idea seems strange.