code: private static String join(String prefix, String suffix) {
          int prefixLength = prefix.length();
          boolean haveSlash = (prefixLength > 0 && prefix.charAt(prefixLength - 1) == separatorChar);
          if (!haveSlash) {
              haveSlash = (suffix.length() > 0 && suffix.charAt(0) == separatorChar);
          }
          return haveSlash ? (prefix + suffix) : (prefix + separatorChar + suffix);
      }
docs: Joins two path components, adding a separator only if necessary.