if (input.equals(" ") || input.equals(null) || input.equals(""))
         {
            do
            {
            input = JOptionPane.showInputDialog("Not a valid first name. "+
                                             "\nClient's " + (i+1) + " first name?");
            }while (input.equals(" ") || input.equals(null) || input.equals(""));
         }
         client[i].setFirstName(input);