TheAlgorithms/Java · error · IllegalArgumentException

Input must be a positive integer. Received: {n}

Error message

Input must be a positive integer. Received: {n}

What it means

Thrown by HarshadNumber.isHarshad(long n) when n is less than or equal to 0. A Harshad (or Niven) number is a positive integer divisible by the sum of its digits, so zero and negatives are excluded by definition. The method computes the digit sum in a while loop and divides n by that sum, so non-positive input would either be meaningless or cause division by zero (digit sum of 0). The error message includes the received value.

Source

Thrown at src/main/java/com/thealgorithms/maths/HarshadNumber.java:30

 *      Wikipedia</a>
 */
public final class HarshadNumber {
    private HarshadNumber() {
    }

    /**
     * Checks if a number is a Harshad number.
     * A Harshad number is a positive integer that is divisible by the sum of its
     * digits.
     *
     * @param n the number to be checked (must be positive)
     * @return {@code true} if {@code n} is a Harshad number, otherwise
     *         {@code false}
     * @throws IllegalArgumentException if {@code n} is less than or equal to 0
     */
    public static boolean isHarshad(long n) {
        if (n <= 0) {
            throw new IllegalArgumentException("Input must be a positive integer. Received: " + n);
        }

        long temp = n;
        long sumOfDigits = 0;
        while (temp > 0) {
            sumOfDigits += temp % 10;
            temp /= 10;
        }

        return n % sumOfDigits == 0;
    }

    /**
     * Checks if a number represented as a string is a Harshad number.
     * A Harshad number is a positive integer that is divisible by the sum of its
     * digits.
     *
     * @param s the string representation of the number to be checked

View on GitHub (pinned to fdfb9a395b)

Solutions

1. Ensure n > 0 before calling isHarshad(long).
2. Validate parsed user input for positivity at the source.
3. Handle zero and negative inputs with domain-appropriate logic (return false or throw a custom exception).

Example fix

// before
boolean result = HarshadNumber.isHarshad(n);

// after
if (n <= 0) {
    return false;
}
boolean result = HarshadNumber.isHarshad(n);

if (n <= 0) {
    return false;
}
boolean result = HarshadNumber.isHarshad(n);

static boolean isPositiveLong(long n) {
    return n > 0;
}

try {
    boolean result = HarshadNumber.isHarshad(n);
} catch (IllegalArgumentException e) {
    result = false; // n <= 0, not a Harshad number
}

When it happens

Trigger: Calling isHarshad(0L), isHarshad(-18L), or any non-positive long. The check fires before any digit-sum computation.

Common situations: Processing parsed integers from user input without positivity checks. Mathematical pipelines that pass through zero or negative values. Test cases for boundary conditions.

Related errors

• Input value must be a positive integer. Input value: {number
• Input string cannot be null or empty
• Input string must be a valid integer: {s}
• Input must be non-negative
• Input 'n' is too big to give accurate result.

AI-assisted analysis of TheAlgorithms/Java@fdfb9a395b (2026-08-13). Data as JSON: /api/errors/0c8cda582eaf4977. Report an issue: GitHub.