code: public static <T> String join(List<T> list, String separator) {
  		StringBuilder builder = new StringBuilder();
  		int i = 0;
  		for (T t : list) {
  			builder.append(t);
  			if (++i < list.size()) builder.append(separator);
  		}
  		return builder.toString();
  	}
docs: Constructs and returns a string object that is the result of interposing a separator between the elements of the list