TheAlgorithms/Java · error · IllegalArgumentException

Input '{input}' contains not only digits

Error message

Input '{input}' contains not only digits

What it means

Thrown by Damm.checkInput when the input string does not match the regex \d+ (one or more decimal digits). The Damm algorithm computes a check digit using a quasigroup operation table and requires a purely numeric input string; any non-digit character (letter, space, dash, sign, decimal point) is rejected. The message echoes the offending input verbatim.

Source

Thrown at src/main/java/com/thealgorithms/others/Damm.java:109

        System.out.println("\nCheck digit generation example:");
        var input = "572";
        generateAndPrint(input);
    }

    private static void checkAndPrint(String input) {
        String validationResult = Damm.dammCheck(input) ? "valid" : "not valid";
        System.out.println("Input '" + input + "' is " + validationResult);
    }

    private static void generateAndPrint(String input) {
        String result = addDammChecksum(input);
        System.out.println("Generate and add checksum to initial value '" + input + "'. Result: '" + result + "'");
    }

    private static void checkInput(String input) {
        Objects.requireNonNull(input);
        if (!input.matches("\\d+")) {
            throw new IllegalArgumentException("Input '" + input + "' contains not only digits");
        }
    }

    private static int[] toIntArray(String string) {
        return string.chars().map(i -> Character.digit(i, 10)).toArray();
    }
}

View on GitHub (pinned to fdfb9a395b)

Solutions

1. Strip non-digit characters and whitespace before calling: input.replaceAll("\\D+", "").
2. Reject or normalize formatted identifiers (remove dashes/spaces) upstream.
3. Validate with your own \d+ check first to give a clearer error context.

Example fix

// before
String id = "123-456-789";
Damm.dammCheck(id); // throws: contains dashes

// after
String id = "123-456-789".replaceAll("\\D+", "");
boolean ok = Damm.dammCheck(id);

String digits = input == null ? "" : input.replaceAll("\\D+", "");
if (digits.isEmpty()) {
    throw new IllegalArgumentException("Input has no digits: " + input);
}
boolean ok = Damm.dammCheck(digits);

static boolean isAllDigits(String s) {
    return s != null && s.matches("\\d+");
}

When it happens

Trigger: Calling Damm.dammCheck/addDammChecksum with a string containing letters, whitespace, a leading sign (+/-), a decimal point, or formatting characters (dashes in a pseudo-account number). An empty string also fails \d+ (which requires at least one digit).

Common situations: Passing a formatted identifier (e.g., "123-456"), a numeric string with a leading '+', a value with surrounding whitespace, or a value that is actually alphanumeric.

Related errors

• Credit card number {" + cardNumber + "} - have a typo
• Input must be a non-empty binary string.
• Input must contain only '0' and '1'. Found: {}
• Input must contain only '0' and '1'.
• Input data contains invalid characters. Only uppercase A-Z a

AI-assisted analysis of TheAlgorithms/Java@fdfb9a395b (2026-08-13). Data as JSON: /api/errors/f6fd692eae5f8617. Report an issue: GitHub.