TheAlgorithms/Java · error · IllegalArgumentException

Credit card number {" + cardNumber + "} - have a typo

Error message

Credit card number {" + cardNumber + "} - have a typo

What it means

Thrown by Luhn.CreditCard.fromString when the input is a valid 16-digit string but fails the Luhn checksum. This indicates a syntactically well-formed number that is not a real/valid card number per the Luhn algorithm (likely a typo).

Source

Thrown at src/main/java/com/thealgorithms/others/Luhn.java:120

        /**
         * @param cardNumber string representation of credit card number - 16
         * digits. Can have spaces for digits separation
         * @return credit card object
         * @throws IllegalArgumentException if input string is not 16 digits or
         * if Luhn check was failed
         */
        public static CreditCard fromString(String cardNumber) {
            Objects.requireNonNull(cardNumber);
            String trimmedCardNumber = cardNumber.replaceAll(" ", "");
            if (trimmedCardNumber.length() != DIGITS_COUNT || !trimmedCardNumber.matches("\\d+")) {
                throw new IllegalArgumentException("{" + cardNumber + "} - is not a card number");
            }

            int[] cardNumbers = toIntArray(trimmedCardNumber);
            boolean isValid = luhnCheck(cardNumbers);
            if (!isValid) {
                throw new IllegalArgumentException("Credit card number {" + cardNumber + "} - have a typo");
            }

            return new CreditCard(cardNumbers);
        }

        /**
         * @return string representation separated by space every 4 digits.
         * Example: "5265 9251 6151 1412"
         */
        public String number() {
            StringBuilder result = new StringBuilder();
            for (int i = 0; i < DIGITS_COUNT; i++) {
                if (i % 4 == 0 && i != 0) {
                    result.append(" ");
                }
                result.append(digits[i]);
            }
            return result.toString();

View on GitHub (pinned to fdfb9a395b)

Solutions

1. Prompt the user to re-enter the number; a Luhn failure almost always means a typo.
2. If generating test data, use a Luhn-valid test card number (e.g. known test PANs).
3. Compute and append the correct check digit programmatically before calling fromString.
4. Double-check for digit transposition in upstream capture (OCR, keypunch).

Example fix

// before
CreditCard cc = CreditCard.fromString(userInput); // 16 digits but fails checksum

// after
// surface a user-facing error and request re-entry
try {
    CreditCard cc = CreditCard.fromString(userInput);
} catch (IllegalArgumentException e) {
    // tell the user the number looks mistyped; do not store it
}

String digits = cardNumber.replaceAll("\\D", "");
if (digits.length() != 16) throw new IllegalArgumentException("not 16 digits");
// Luhn pre-check (mirrors the library) to fail fast with context
int sum = 0; boolean alt = false;
for (int i = digits.length() - 1; i >= 0; i--, alt = !alt) {
    int d = digits.charAt(i) - '0';
    if (alt) { d *= 2; if (d > 9) d -= 9; }
    sum += d;
}
if (sum % 10 != 0) throw new IllegalArgumentException("fails Luhn checksum");

public static boolean passesLuhn(String digits) {
    int sum = 0; boolean alt = false;
    for (int i = digits.length() - 1; i >= 0; i--, alt = !alt) {
        int d = digits.charAt(i) - '0';
        if (alt) { d *= 2; if (d > 9) d -= 9; }
        sum += d;
    }
    return sum % 10 == 0;
}

try {
    cc = CreditCard.fromString(raw);
} catch (IllegalArgumentException e) {
    if (e.getMessage().contains("have a typo")) {
        errors.add("Card number looks mistyped; please re-enter.");
    } else {
        throw e;
    }
}

When it happens

Trigger: Calling CreditCard.fromString(s) where s is 16 digits but the Luhn check (luhnCheck) returns false; a single mistyped digit, a transposed pair of digits, or a fabricated 16-digit number.

Common situations: Manual data entry typo; OCR scan with one wrong digit; test fixture with a made-up number that happens to be 16 digits; transposed adjacent digits from copy-paste.

Related errors

• {" + cardNumber + "} - is not a card number
• Input '{input}' contains not only digits
• Position must be between 0 and {array.length}
• Array is empty
• Position must be between 0 and + (array.length - 1)

AI-assisted analysis of TheAlgorithms/Java@fdfb9a395b (2026-08-13). Data as JSON: /api/errors/630bb7e56df98edf. Report an issue: GitHub.