code: public final void addString(String key, String... values) {
          if (values == null || Array.getLength(values) == 0) {
              return;
          }
          List<String> list = parameters.get(key);
          if (list == null) {
              list = new ArrayList<>();
          }
          list.addAll(Arrays.asList(values));
          parameters.put(key, list);
      }
docs: Adds one or multiple string values.
  Adding a single <code>null</code> value has no effect.
  When adding multiple values, <code>null</code> values are converted
  to blank strings.
  @param key the key of the value to set
  @param values the values to set