code: public File createNewFile() throws IOException {
          if (FileLogUtils.validateDirectory(directory) == null) {
              return null;
          }
  
          return rolling ? rollFile() : createNewUniqueFile(null);
      }
docs: Creates a new file for this log set. If this is a rolling log, this will
  rename the existing file and then return it on success. Otherwise, this
  will return a new unique file.
  
  @return the file on success, or null on failure
  @throws IOException