code: public String get(String key) throws BadRequestException {
  
      List<String> list = this.parameters.get(key);
      if ((list == null) || (list.isEmpty())) {
        return null;
      }
      if (list.size() > 1) {
        throw new BadRequestException("Duplicate parameter: " + key);
      }
      return list.get(0);
    }
docs: Gets the parameter as single value with the given <code>key</code> as {@link String}.
  
  @param key is the {@link java.util.Map#get(Object) key} of the parameter to get.
  @return the requested parameter. Will be {@code null} if the parameter is not present.
  @throws BadRequestException if the parameter is defined multiple times (see {@link #getList(String)}).