The javadoc documentation says

     In fact, Javadoc will run on .java source files that are pure
     stub files with no method bodies. This means you can write
     documentation comments and run Javadoc in the earliest stages
     of design while creating the API, before writing the
     implementation.

In fact, javadoc complains if you try to do this, and insists that
you add "abstract" to the methods and the class.