code: private int shiftMask(FileAccessClass fileModeClass, int bitMask) {
  
      if (fileModeClass == FileAccessClass.USER) {
        return bitMask << 6;
      } else if (fileModeClass == FileAccessClass.GROUP) {
        return bitMask << 3;
      } else if (fileModeClass == FileAccessClass.OTHERS) {
        return bitMask;
      }
      throw new IllegalArgumentException("Illegal FileModeClass: " + fileModeClass);
    }
docs: This method shifts the given {@code bitMask} according to the given {@code fileModeClass}.
  
  @param fileModeClass is the class of access ( {@link FileAccessClass#USER}, {@link FileAccessClass#GROUP}, or
  {@link FileAccessClass#OTHERS}).
  @param bitMask is the bit-mask to shift.
  @return the shifted {@code bitMask}.