code: public static String validateRequired(final InputComponent<?, ?> input)
     {
        String requiredMessage = null;
        if (input.isEnabled() && input.isRequired() && !InputComponents.hasValue(input))
        {
           requiredMessage = input.getRequiredMessage();
           if (Strings.isNullOrEmpty(requiredMessage))
           {
              String labelValue = input.getLabel() == null ? input.getName() : input.getLabel();
              // Chop : off
              if (labelValue.endsWith(COLON))
              {
                 labelValue = labelValue.substring(0, labelValue.length() - 1);
              }
              requiredMessage = labelValue + " must be specified.";
           }
        }
        return requiredMessage;
     }
docs: Validate if an required and enabled input has a value. If not, return the error message.
  
  @param input
  @return