TheAlgorithms/Java · error · IllegalArgumentException

Start must be non-negative. Given start:

Error message

Start must be non-negative. Given start: 

What it means

Thrown by KaprekarNumbers.kaprekarNumberInRange(long start, long end) when start is negative. Kaprekar numbers are defined for non-negative integers (0 and 1 are considered Kaprekar by isKaprekarNumber). This check runs after the start > end check (error 418), so a negative start that is also > end would hit the range error first. The error message appends the start value. Note: the Javadoc says 'start is negative' but only start is checked — end is implicitly allowed to be any value >= start.

Source

Thrown at src/main/java/com/thealgorithms/maths/KaprekarNumbers.java:48

public final class KaprekarNumbers {
    private KaprekarNumbers() {
    }

    /**
     * Finds all Kaprekar numbers within a given range (inclusive).
     *
     * @param start the starting number of the range (inclusive)
     * @param end   the ending number of the range (inclusive)
     * @return a list of all Kaprekar numbers in the specified range
     * @throws IllegalArgumentException if start is greater than end or if start is
     *                                  negative
     */
    public static List<Long> kaprekarNumberInRange(long start, long end) {
        if (start > end) {
            throw new IllegalArgumentException("Start must be less than or equal to end. Given start: " + start + ", end: " + end);
        }
        if (start < 0) {
            throw new IllegalArgumentException("Start must be non-negative. Given start: " + start);
        }

        ArrayList<Long> list = new ArrayList<>();
        for (long i = start; i <= end; i++) {
            if (isKaprekarNumber(i)) {
                list.add(i);
            }
        }

        return list;
    }

    /**
     * Checks whether a given number is a Kaprekar number.
     * <p>
     * The algorithm works as follows:
     * <ol>
     * <li>Square the number</li>

View on GitHub (pinned to fdfb9a395b)

Solutions

1. Ensure start >= 0 before calling kaprekarNumberInRange().
2. Clamp start to 0 if negative values may occur: start = Math.max(0, start).
3. Validate both range bounds at the input boundary before invoking.

Example fix

// before
List<Long> result = KaprekarNumbers.kaprekarNumberInRange(start, end);

// after
long s = Math.max(0, Math.min(start, end));
long e = Math.max(start, end);
List<Long> result = KaprekarNumbers.kaprekarNumberInRange(s, e);

if (start < 0) {
    throw new IllegalArgumentException("start must be non-negative: " + start);
}
List<Long> result = KaprekarNumbers.kaprekarNumberInRange(start, end);

static boolean isNonNegativeStart(long start, long end) {
    return start >= 0 && start <= end;
}

try {
    List<Long> result = KaprekarNumbers.kaprekarNumberInRange(start, end);
} catch (IllegalArgumentException e) {
    if (e.getMessage().contains("non-negative")) {
        // start < 0; clamp to 0
    }
}

When it happens

Trigger: Calling kaprekarNumberInRange(-5, 10), kaprekarNumberInRange(-100, -1) (but -100 > -1 is false, so the range check passes and this fires), or any case where start < 0 and start <= end. The internal isKaprekarNumber also checks for negative num, so without this range-level guard, individual negative values would throw deeper.

Common situations: User-supplied ranges starting from negative numbers. Data boundaries computed from offsets that can go below zero. Configuration with unvalidated lower bounds.

Related errors

• Start must be less than or equal to end. Given start: {start
• Input must be non-negative!
• Input 'n' is too big to give accurate result.
• k must be between 1 and the size of the array
• Array must be non-empty.

AI-assisted analysis of TheAlgorithms/Java@fdfb9a395b (2026-08-13). Data as JSON: /api/errors/b780f5696f8e8f29. Report an issue: GitHub.