code: public static String joinStrings( String separator, String... strings ) {
          if (separator == null) {
              separator = "";
          }
          StringBuilder sb = new StringBuilder();
          for( int i = 0; i < strings.length; i++ ) {
              sb.append(strings[i]);
              if (i < strings.length - 1) {
                  sb.append(separator);
              }
          }
          return sb.toString();
      }
docs: Join strings through {@link StringBuilder}.
  
  @param separator separator to use or <code>null</code>.
  @param strings strings to join.
  @return the joined string.