code: private boolean checkParameterSet(HttpServletResponse resp, String parameter, String parameterName)
  			throws IOException {
  		if (parameter == null) {
  			showErrorPage(resp, "The parameter '" + parameterName + "' is not set.");
  			return true;
  		} else {
  			return false;
  		}
  	}
docs: Checks the given parameter. If it is null, it prints the error page.
  
  @param resp
  The response handler.
  @param parameter
  The parameter to check.
  @param parameterName
  The name of the parameter, to describe it in the error message.
  @return True, if the parameter is null, false otherwise.
  @throws IOException
  Thrown, if the error page could not be printed.