public CommandConstraintException(Set<ConstraintViolation<ICommand>> constraintViolations) {
   if (UtilValidator.isNotEmpty(constraintViolations)) {
     for (ConstraintViolation constraintViolation : constraintViolations) {
       constraintMessages.add(constraintViolation.getMessage());
     }
     this.constraintViolations = constraintViolations;
   }
 }
 public void issueBooks() {
   List<Map<String, ?>> booksToBeIssued = ViewUtil.getSelectedItems(availableBooksListBox);
   if (UtilValidator.isNotEmpty(booksToBeIssued)) issueBooks(getBookIds(booksToBeIssued));
 }
 public void returnBooks() {
   List<Map<String, ?>> booksToBeReturned = ViewUtil.getSelectedItems(booksWithMemberListBox);
   if (UtilValidator.isNotEmpty(booksToBeReturned)) returnBooks(getBookIds(booksToBeReturned));
 }