code: void add(final File file) {
      if (file.isFile()) {
        if (this.fileNames.contains(file.getName())) {
          LOG.log(Level.FINEST, "A file with this name has already been added: {0}", file.getName());
        } else {
          this.fileNames.add(file.getName());
          this.theFiles.add(file);
        }
      } else {
        LOG.log(Level.FINEST, "Ignoring, because it is not a proper file: {0}", file);
      }
    }
docs: Add a file to the FileSet.
  <p>
  If the file is a directory, it is turned into a JAR and the resulting JAR is added.
  <p>
  Files already added will be ignored.
  
  @param file the file to be added.