code: public static String normalizeSeparators(final String filename) {
          if (filename == null) {
              return null;
          }
          switch (SYSTEM_SEPARATOR) {
          case UNIX_SEPARATOR:
              return filename.replace(WINDOWS_SEPARATOR, UNIX_SEPARATOR);
          case WINDOWS_SEPARATOR:
              return filename.replace(UNIX_SEPARATOR, WINDOWS_SEPARATOR);
          default:
              throw new IllegalStateException("Unexpected file separator");
          }
      }
docs: Returns a normalized filename, where the incoming filename has backslashes converted to
  forward slashes (on Unix OS), and vice-versa on Windows OS, otherwise, the path separation
  methods of {@link File} will not work.
  
  Note that unlike {@link FilenameUtils#normalize(String)}, this method does not change or
  remove any relative paths, etc, which could cause the return value to become null.
  
  @param filename
  filename whose separators should be normalized
  @return filename with normalized separators