How can i resolve the error Cannot cast java.lang.Integer to java.lang.String?

I am implementing the process of borrowing a book using angular and springboot but I am getting the error Cannot cast java.lang.Integer to java.lang.String.

Here is my Angular code:

borrowBook(book: BookResponse) {
this.message = "";
this.bookService.borrowBook({
'book-id': book.id as number
}).subscribe({
next: () => {
this.level = 'success';
this.message = "Book successfully added to your list";
},
error: (err) => {
console.log(err);
this.level = 'error';
this.message = err.error.error;
}
});
}


In Spring boot, this is my controller method :

@PostMapping("borrow/{book-id}")
public ResponseEntity<Integer> borrowBook(
@PathVariable("book-id") Integer bookId,
Authentication connectedUser
) {
return ResponseEntity.ok(bookService.borrowBook(bookId, connectedUser));
}


The logic is as below:

public Integer borrowBook(Integer bookId, Authentication connectedUser) {
Book book = bookRepository.findById(bookId)
.orElseThrow(() -> new EntityNotFoundException("No book found with ID:: " + bookId));
if (book.isArchived() || !book.isShareable()) {
throw new OperationNotPermittedException("The requested book cannot be borrowed since it is archived or not shareable");
}
User user = ((User) connectedUser.getPrincipal());
if (Objects.equals(book.getOwner().getUser_id(), user.getUser_id())) {
throw new OperationNotPermittedException("You cannot borrow your own book");
}

System.out.println("this works");
final boolean isAlreadyBorrowedByUser = transactionHistoryRepository.isAlreadyBorrowedByUser(bookId, user.getUser_id());
if (isAlreadyBorrowedByUser) {
throw new OperationNotPermittedException("You already borrowed this book and it is still not returned or the return is not approved by the owner");
}
final boolean isAlreadyBorrowedByOtherUser = transactionHistoryRepository.isAlreadyBorrowed(bookId);
if (isAlreadyBorrowedByOtherUser) {
throw new OperationNotPermittedException("The requested book is already borrowed");
}

BookTransactionHistory bookTransactionHistory = BookTransactionHistory.builder()
.user(user)
.book(book)
.isReturned(false)
.isReturnApproved(false)
.build();
return transactionHistoryRepository.save(bookTransactionHistory).getId();

}


Here is my repository :

@Query("""
SELECT CASE WHEN COUNT(bookTransactionHistory) > 0 THEN true ELSE false END
FROM BookTransactionHistory bookTransactionHistory
WHERE bookTransactionHistory.user.user_id = :userId
AND bookTransactionHistory.book.id = :bookId
AND bookTransactionHistory.isReturnApproved = false
""")
boolean isAlreadyBorrowedByUser(@Param("bookId") Integer bookId, @Param("userId") Integer userId);

@Query("""
SELECT CASE WHEN COUNT(bookTransactionHistory) > 0 THEN true ELSE false END
FROM BookTransactionHistory bookTransactionHistory
WHERE bookTransactionHistory.book.id = :bookId
AND bookTransactionHistory.isReturnApproved = false
""")
boolean isAlreadyBorrowed(@Param("bookId") Integer bookId);


And the book entity is as follows:

@Getter
@Setter
@SuperBuilder
@AllArgsConstructor
@NoArgsConstructor
@Entity
public class BookTransactionHistory extends BaseEntity {

// user relationship
@ManyToOne
@JoinColumn(name = "user_id")
private User user;

// book relationship
@ManyToOne
@JoinColumn(name = "book_id")
private Book book;


private boolean isReturned;
private boolean isReturnApproved;
}


The above code is expected to report errors when a user borrows a book they own, they have already borrowed or have been borrowed by other users, otherwise return the id of the borrowed book. However, I am experiencing the above error, and I am thinking it has to do with the @Query in the repository. I have tried changing the borrowBook() return type to string but then i would have to return another field other than the id.

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