code: public static String join(@Nullable List<String> tokens, @Nonnull String delimiter) {
          if (tokens == null) {
              return null;
          }
          Preconditions2.checkNotNull(delimiter, "given delimiter can not be null");
  
          Iterator<String> it = tokens.iterator();
          StringBuilder sb = new StringBuilder();
          while (it.hasNext()) {
              String token = it.next();
              sb.append(token);
              if (it.hasNext()) {
                  sb.append(delimiter);
              }
          }
          return sb.toString();
      }
docs: Join given {@code tokens} with given {@code delimiter}.
  
  Sample: tokens {@code "com", "mycompany, "ecommerce", "server1"} with delimiter {@code "."}
  returns {@code "com.mycompany.ecommerce.server1"}.
  
  @param tokens
  @param delimiter
  @return the joined tokens ({@code null} if given {@code tokens} is {@code null}