code: public void putStage(String stage) throws JSONException {
          if (stage != null) {
              this.stage = stage;
              this.put(Defines.LinkParam.Stage.getKey(), stage);
          }
      }
docs: <b>A string value that represents the stage of the user in the app. eg:
  "level1", "logged_in", etc.</b>
  
  @param stage A {@link String} value specifying the stage.
  @throws JSONException The parameter value must be in valid JSON format, or a
  {@link JSONException} will be thrown.