Header files are a necessity in C/C++/ObjC because each file needs the definition of all its symbols before being compiled into an object file. One side-effect is that a library distributor that do not want to open its source code can provide a client only with a header file and the .o to be linked. Is this possible in languages that rely on a full view of the source code during compilation, like Java? Are there any other languages with interesting solutions for this use case?
To provide a bit of context: In my (5y+) experience as a software engineer, I've only ever used libraries to which I have direct source code access in Java, Python and Go, and have never developed closed-source libraries. For the first two a closed-source developer could ship bytecode, but I don't see how it's possible to do that with Go, which doesn't rely on header files to forward-declare symbols.
Is this possible in languages that rely on a full view of the source code during compilation, like Java?
Bzzt. Java does not 'rely on a full view of the source code during compilation'. You don't need the source code to use a Java JAR file, right? The export symbol information is in the .class file. Consider the JDK's rt.jar as the most trivial example. If that didn't work, nothing would. And you don't need the source code for that.
In Modula, Ada, etc, it is also in the object code, somehow.
It is really just about only C and C++ that don't have this feature, and therefore must rely on header files.