code: private void setFlag(FileAccessClass fileModeClass, int bitMask, boolean flag) {
  
      setBits(shiftMask(fileModeClass, bitMask), flag);
    }
docs: This method sets the flag(s) given by {@code bitMask} of this this {@link #getMaskBits() mask} for the given
  {@code fileModeClass} to the given value ({@code flag}).
  
  
  @param fileModeClass is the class of access ( {@link FileAccessClass#USER}, {@link FileAccessClass#GROUP}, or
  {@link FileAccessClass#OTHERS}).
  @param bitMask is the bit-mask of the flag(s) to set.
  @param flag - if {@code true} the flag will be set, if {@code false} it will be unset.