How to add "SUM" to Due amount Attribute in order management extension message

Hello

we need to update "SUM" to Due amount Attribute in order management extension message

String messageText = "Outstanding Invoice Exists for the customer and payment schedule:"+ prow.getAttribute("PaymentScheduleDueDate")+sum("Due:")+prow.getAttribute("PaymentScheduleAmountDueRemaining");

throw new ValidationException( new Message(Message.MessageType.WARNING,messageText) );

}

}}