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If I replace a class with a private constructor by an interface… AFAIK this is a source-compatible change, but is it a binary-compatible change?

with only a private constructor? that doesn't sound like a useful class

It has members, but I would keep their signature identical in the interface. My point is, external users couldn't instantiate it anyway, so is the class/interface somewhat relevant at the binary level, which would make this a breaking binary change even though it's source-safe?

on JVM class member methods are called with the `invokevirtual` bytecode instruction while interface methods are called with `invokeinterface`. there's some subtle difference between the two. I can't think of anything that will outright *break* due to the difference, I don't think the verifier cares… but I'm not sure

Hm, so at the very best it's still quite shady. Thanks for the info

Got curious and tried it out, I got
```Exception in thread "main" java.lang.IncompatibleClassChangeError: Found interface a.A, but class was expected
	at main.Main.main(Main.java:7)```

Can’t find any reference of it in the spec though

Source code is <https://github.com/openjdk/jdk/blob/383fe6eaab423a1218c9915362f691472e3773e7/src/hotspot/share/interpreter/linkResolver.cpp#L751|here> if you fancy cpp :slightly_smiling_face:

ah, I didn't check the spec. it does mention this, <https://docs.oracle.com/javase/specs/jvms/se8/html/jvms-5.html#jvms-5.4.3.3>
&gt; When resolving a method reference: If C is an interface, method resolution throws an `IncompatibleClassChangeError`.

Thanks a lot, that's very useful to know! Not the answer I would have preferred, but oh well :sweat_smile: