TheAlgorithms/Java · error · IllegalArgumentException

Input '" + input + "' contains not only digits

Error message

Input '" + input + "' contains not only digits

What it means

Thrown by Verhoeff.checkInput (invoked by verhoeffCheck and addVerhoeffChecksum) when the input string does not match the regex \d+, i.e. contains any non-digit character or is empty. The Verhoeff algorithm operates purely on decimal digits via dihedral-group multiplication and permutation tables, so letters, spaces, dashes, or a blank string are invalid inputs.

Source

Thrown at src/main/java/com/thealgorithms/others/Verhoeff.java:164

        System.out.println("\nCheck digit generation example:");
        var input = "236";
        generateAndPrint(input);
    }

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

    private static void generateAndPrint(String input) {
        String result = addVerhoeffChecksum(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 all non-digit characters from the input before calling: input.replaceAll("\\D+", "").
2. Reject or re-prompt empty input upstream so the algorithm never receives "".
3. Validate with input.matches("\\d+") (or Character.isDigit on each char) before invoking and surface a user-facing validation error instead of letting the exception propagate.
4. Ensure the input has at least one digit; note that \d+ does not match the empty string.

Example fix

// before
boolean ok = Verhoeff.verhoeffCheck("123-456");
// after
String clean = "123-456".replaceAll("\\D+", "");
boolean ok = clean.isEmpty() ? false : Verhoeff.verhoeffCheck(clean);

String clean = input == null ? "" : input.replaceAll("\\D+", "");
if (clean.isEmpty()) {
    // handle empty input in UI/parse layer
    return;
}
boolean ok = Verhoeff.verhoeffCheck(clean);

static boolean isAllDigits(String s) {
    return s != null && !s.isEmpty() && s.chars().allMatch(Character::isDigit);
}

try {
    boolean ok = Verhoeff.verhoeffCheck(raw);
} catch (IllegalArgumentException e) {
    // surface a user-facing "invalid format" message
}

When it happens

Trigger: Calling verhoeffCheck or addVerhoeffChecksum with a string containing hyphens ("123-45"), spaces, letters, a plus sign, a leading +, or an empty string "" (\d+ requires at least one digit).

Common situations: Passing a formatted identification number with separators (e.g. ISBN/Aadhaar-style "1234 5678 9012"), user input with whitespace not trimmed, copy-pasted values containing non-breaking spaces, or an empty form field.

Related errors

• Invalid unit '{}'. Supported units are: {}
• inputUnit must be different from outputUnit.
• NULL_INPUT
• UNKNOWN_WORD
• Cannot enqueue null data

AI-assisted analysis of TheAlgorithms/Java@fdfb9a395b (2026-08-13). Data as JSON: /api/errors/800418187a10d3d1. Report an issue: GitHub.