How to Validate an ISBN in JPA?

An ISBN (International Standard Book Number) is a 10-digit or 13-digit number that uniquely identifies books.
To validate an ISBN, we need to check its length and checksum.

📌 Rules for ISBN Validation

1. ISBN-10 Rules:
   • Contains 10 digits (0-9).
   • The last digit (checksum) can be a digit (0-9) or X.
   • Checksum formula:
     [ (1×d_1 + 2×d_2 + … + 10×d_{10}) \mod 11 = 0 ]
   • Example: 0-306-40615-2 ✅ (valid)
2. ISBN-13 Rules:
   • Contains 13 digits (0-9).
   • Checksum formula:
     [ (1×d_1 + 3×d_2 + 1×d_3 + 3×d_4 + … + 3×d_{12} + d_{13}) \mod 10 = 0 ]
   • Example: 978-3-16-148410-0 ✅ (valid)

📌 Java Implementation: Validate ISBN-10 and ISBN-13

public class ISBNValidator {

    public static boolean isValidISBN(String isbn) {
        if (isbn == null) {
            return false;
        }
        
        isbn = isbn.replace("-", "").replace(" ", ""); // Remove hyphens and spaces

        if (isbn.length() == 10) {
            return isValidISBN10(isbn);
        } else if (isbn.length() == 13) {
            return isValidISBN13(isbn);
        } else {
            return false; // Invalid length
        }
    }

    private static boolean isValidISBN10(String isbn) {
        if (!isbn.matches("\\d{9}[\\dX]")) {
            return false; // Must be 9 digits + 1 digit/X
        }

        int sum = 0;
        for (int i = 0; i < 9; i++) {
            sum += (isbn.charAt(i) - '0') * (i + 1);
        }

        char lastChar = isbn.charAt(9);
        int lastDigit = (lastChar == 'X') ? 10 : (lastChar - '0');

        sum += lastDigit * 10;

        return sum % 11 == 0;
    }

    private static boolean isValidISBN13(String isbn) {
        if (!isbn.matches("\\d{13}")) {
            return false; // Must be exactly 13 digits
        }

        int sum = 0;
        for (int i = 0; i < 12; i++) {
            int digit = isbn.charAt(i) - '0';
            sum += (i % 2 == 0) ? digit : digit * 3;
        }

        int checksum = (10 - (sum % 10)) % 10;

        return checksum == (isbn.charAt(12) - '0');
    }

    public static void main(String[] args) {
        System.out.println(isValidISBN("0-306-40615-2")); // true (valid ISBN-10)
        System.out.println(isValidISBN("978-3-16-148410-0")); // true (valid ISBN-13)
        System.out.println(isValidISBN("123456789X")); // true (valid ISBN-10)
        System.out.println(isValidISBN("9783161484100")); // true (valid ISBN-13)
        System.out.println(isValidISBN("1234567890")); // false (invalid ISBN-10)
        System.out.println(isValidISBN("9783161484105")); // false (invalid ISBN-13)
    }
}

📌 How to Use This in a JPA Entity?

Add the validation logic in the Book entity:

@Entity
@Getter
@Setter
@NoArgsConstructor
@AllArgsConstructor
@Builder
public class Book {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String title;
    private String author;

    @Column(unique = true)
    private String isbn;

    private int publishedYear;

    @PrePersist
    @PreUpdate
    private void validateISBN() {
        if (!ISBNValidator.isValidISBN(this.isbn)) {
            throw new IllegalArgumentException("Invalid ISBN: " + this.isbn);
        }
    }
}