code: public static String join(Collection<?> objectsToJoin, String separator) {
          if ((objectsToJoin != null) && !objectsToJoin.isEmpty()) {
              StringBuilder builder = new StringBuilder();
              for (Object object : objectsToJoin) {
                  builder.append(object);
                  builder.append(separator);
              }
              // Remove the last separator
              return builder.substring(0, builder.length() - separator.length());
          } else {
              return EMPTY;
          }
      }
docs: Joins all the strings in the list in a single one separated by the separator sequence.
  
  @param objectsToJoin The objects to join.
  @param separator     The separator sequence.
  @return The joined strings.