code: public static String join(String delimiter, String... parts) {
      if (parts == null) {
        return null;
      }
  
      if (delimiter == null) {
        delimiter = "";
      }
  
      StringBuilder stringBuilder = new StringBuilder();
      for (int i=0; i < parts.length; i++) {
        if (i > 0) {
          stringBuilder.append(delimiter);
        }
        stringBuilder.append(parts[i]);
      }
      return stringBuilder.toString();
    }
docs: Joins a list of Strings to a single one.
  
  @param delimiter the delimiter between the joined parts
  @param parts the parts to join
  @return the joined String or null if parts was null