code: public static String join(List<String> strings) {
          StringBuilder result = new StringBuilder();
  
          boolean first = true;
          for (String s : strings) {
              if (!first) result.append(", ");
  
              result.append(s);
              first = false;
          }
  
          return result.toString();
      }
docs: Returns a new string created by joining each of the strings in the
  specified list together, with a comma between them.
  
  @param strings
  The list of strings to join into a single, comma delimited
  string list.
  @return A new string created by joining each of the strings in the
  specified list together, with a comma between strings.