code: public S setParameter(String key, String value) {
          if (parameters == null) {
              parameters = new ConcurrentHashMap<String, String>();
          }
          if (value == null) {
              parameters.remove(key);
          } else {
              parameters.put(key, value);
          }
          return castThis();
      }
docs: Sets parameter.
  
  @param key   the key
  @param value the value
  @return the parameter