TheAlgorithms/Java · error · IllegalArgumentException

Input must be non-negative

Error message

Input must be non-negative

What it means

Thrown by CountSetBits.countSetBits(n) when n is negative. The method counts set bits across all integers from 1 to n using a logarithmic formula based on the largest power of two <= n; that formula only holds for non-negative n. Negative inputs are rejected rather than producing wrong results.

Source

Thrown at src/main/java/com/thealgorithms/bitmanipulation/CountSetBits.java:25

 * @author navadeep
 */
public final class CountSetBits {

    private CountSetBits() {
        // Utility class, prevent instantiation
    }

    /**
     * Counts total number of set bits in all numbers from 1 to n
     * Time Complexity: O(log n)
     *
     * @param n the upper limit (inclusive)
     * @return total count of set bits from 1 to n
     * @throws IllegalArgumentException if n is negative
     */
    public static int countSetBits(int n) {
        if (n < 0) {
            throw new IllegalArgumentException("Input must be non-negative");
        }

        if (n == 0) {
            return 0;
        }

        // Find the largest power of 2 <= n
        int x = largestPowerOf2InNumber(n);

        // Total bits at position x: x * 2^(x-1)
        int bitsAtPositionX = x * (1 << (x - 1));

        // Remaining numbers after 2^x
        int remainingNumbers = n - (1 << x) + 1;

        // Recursively count for the rest
        int rest = countSetBits(n - (1 << x));

View on GitHub (pinned to fdfb9a395b)

Solutions

1. Ensure n >= 0 before calling; n == 0 is valid and yields 0.
2. Clamp n to 0 if a non-negative fallback is acceptable, or reject negative input upstream.
3. Check the computation that produced n for underflow.

Example fix

// before
CountSetBits.countSetBits(len - offset);  // throws if offset > len

// after
int n = Math.max(0, len - offset);
CountSetBits.countSetBits(n);

if (n < 0) throw new IllegalArgumentException("n must be >= 0");
CountSetBits.countSetBits(n);

public static boolean isNonNegative(int n) {
    return n >= 0;
}

try {
    return CountSetBits.countSetBits(n);
} catch (IllegalArgumentException e) {
    return 0;
}

When it happens

Trigger: Calling `countSetBits(-1)` or `countSetBits(-100)`. The guard `n < 0` triggers; n == 0 returns 0 (valid), and any positive n is processed. The largestPowerOf2InNumber helper is only invoked for n > 0.

Common situations: n derived from a subtraction that underflows; n read from signed input that can be negative; off-by-one where n represents a size passed as size-1 that went negative.

Related errors

• Invalid BCD digit: {}
• Shift amount cannot be negative: {}
• Bit positions must be between 0 and 31
• Alpha must be between 0 and 1.
• order must be greater than zero

AI-assisted analysis of TheAlgorithms/Java@fdfb9a395b (2026-08-13). Data as JSON: /api/errors/46bd6c163f5b42e7. Report an issue: GitHub.