code: public String joining(CharSequence delimiter) {
          return map(String::valueOf).rawCollect(Collectors.joining(delimiter));
      }
docs: Returns a {@link String} which is the concatenation of the results
  of calling {@link String#valueOf(Object)} on each element of this stream,
  separated by the specified delimiter, in encounter order.
  
  <p>
  This is a <a href="package-summary.html#StreamOps">terminal</a>
  operation.
  
  @param delimiter the delimiter to be used between each element
  @return the result of concatenation. For empty input stream empty String
  is returned.