code: public void putFeature(String feature) throws JSONException {
          if (feature != null) {
              this.feature = feature;
              this.put(Defines.LinkParam.Feature.getKey(), feature);
          }
      }
docs: <b>[Optional] The feature in which the link will be used. eg: "invite",
  "referral", "share", "gift", etc.</b>
  
  @param feature A {@link String} specifying the feature. (max 128 characters).
  @throws JSONException The parameter value must be in valid JSON format, or a
  {@link JSONException} will be thrown.