public String getString(final String key) {
    if (containsKey(key)) {
      return get(key);
    } else {
      throw new UndefinedPropertyException("Missing required property '" + key
          + "'");
    }
  }
Gets the string from the Props. If it doesn't exist, throw and UndefinedPropertiesException
dss-appconn/appconns/dss-eventchecker-appconn/src/main/java/com/webank/wedatasphere/dss/appconn/eventchecker/utils/Props.java