code: public static String concatPath(Object base, Object path) throws IllegalArgumentException {
      Preconditions.checkNotNull(base, "base");
      Preconditions.checkNotNull(path, "path");
      String trimmedBase = SEPARATOR_MATCHER.trimTrailingFrom(base.toString());
      String trimmedPath = SEPARATOR_MATCHER.trimFrom(path.toString());
  
      StringBuilder output = new StringBuilder(trimmedBase.length() + trimmedPath.length() + 1);
      output.append(trimmedBase);
      if (!trimmedPath.isEmpty()) {
        output.append(AlluxioURI.SEPARATOR);
        output.append(trimmedPath);
      }
  
      if (output.length() == 0) {
        // base must be "[/]+"
        return AlluxioURI.SEPARATOR;
      }
      return output.toString();
    }
docs: Joins two path elements, separated by {@link AlluxioURI#SEPARATOR}.
  <p>
  Note that empty element in base or paths is ignored.
  
  @param base base path
  @param path path element to concatenate
  @return joined path
  @throws IllegalArgumentException if base or paths is null