code: public static String join(String separator, String[] parts) {
          StringBuilder builder = new StringBuilder();
          boolean first = true;
          for (String part : parts) {
              if (first) {
                  first = false;
              } else {
                  builder.append(separator);
              }
              builder.append(part);
          }
          return builder.toString();
      }
docs: Joins an array of strings together using a given separator
  @param separator The separator to use
  @param parts An array of strings
  @return The concatenation of the parts with separators in between