> C uses conventions to produce an "interface" (ala a header file with declarations). Java uses compilers to produce an interface, which i do really like. You can ship that interface without an implementation, and only at runtime load an implementation for example.

You can do this in C as well, since it has separate code declarations and definitions. You conventionally put the declarations in the header and definitions in the source file. A C program can link against declarations alone, and the implementations can be loaded later using dynamic linking.

> And these are all othorgonal concerns. A private interface is for the internal organization of code, as opposed to a public one (for external consumption). That's why you might have a private interface.

But these are profoundly overlapping concerns. Interfaces also hide the internal organization of the code, and on top of this, you also have project jigzaw's modules (that absolutely nobody uses), but which also caters toward separating private implementations from public interfaces.