jar

The tool you use to package up class files in Java is called jar. The output from this tool is a "JAR file."

To make a JAR file, first compile your code into a folder.

If you do this by listing all the files out one-by-one, your output folder will be structured something like the following.

output/
  some_pkg/
    A.class
    nested/
      B.class

If you did it using the --module-source-path your class files will be nested under a folder with the name of the module.

output/
  ex.mod/
    module-info.class
    some_pkg/
      A.class
      nested/
        B.class

Then use a command like the following:

jar \
    --create \
    --file ex.mod.jar \
    -C output/ex.mod .

The --create part means "we are creating a new jar file" and --file says what file to put the jar in. For the file name you should generally use the name of the module followed by .jar.¹ This is technically optional though.

The -C flag is where it gets interesting. The jar tool works somewhat like how your terminal does when you cd into directories.

The tool starts at the directory you are running it at.

project/ <--- jar "is here"
  output/
    ex.mod/
      module-info.class
     some_pkg/
        A.class
        nested/
          B.class

Then with -C you "Change" into a directory.

project/ 
  output/
    ex.mod/ <--- jar is here after "-C output/ex.mod"
      module-info.class
     some_pkg/
        A.class
        nested/
          B.class

After that you are meant to specify what files you want to add into the jar. Just putting one period (.) means "take everything in this folder." Hence -C output/ex.mod . puts all those files into the final JAR.