TheAlgorithms/Java · error · IllegalArgumentException

The exponent must be positive

Error message

The exponent must be positive

What it means

Thrown by ModuloPowerOfTwo.moduloPowerOfTwo(int x, int n) when n <= 0. The method computes x mod 2^n via the bit trick x & ((1 << n) - 1). For n = 0 the mask degenerates to 0 (always-yielding 0, not the intended modulo), and for n < 0 the shift is undefined for this trick, so the library requires a strictly positive exponent.

Source

Thrown at src/main/java/com/thealgorithms/bitmanipulation/ModuloPowerOfTwo.java:23

 * of a number when divided by a power of two (2^n)
 * without using division or modulo operations.
 *
 * @author Hardvan
 */
public final class ModuloPowerOfTwo {
    private ModuloPowerOfTwo() {
    }

    /**
     * Computes the remainder of a given integer when divided by 2^n.
     *
     * @param x the input number
     * @param n the exponent (power of two)
     * @return the remainder of x divided by 2^n
     */
    public static int moduloPowerOfTwo(int x, int n) {
        if (n <= 0) {
            throw new IllegalArgumentException("The exponent must be positive");
        }

        return x & ((1 << n) - 1);
    }
}

View on GitHub (pinned to fdfb9a395b)

Solutions

1. Ensure the exponent is at least 1 before calling (n >= 1).
2. Recompute the exponent source so it cannot reach 0 (e.g. require size >= 2).
3. If you need modulo by 1 (result always 0), short-circuit that case explicitly before calling.

Example fix

// before
int r = ModuloPowerOfTwo.moduloPowerOfTwo(x, log2(size));

// after
int exp = Integer.numberOfTrailingZeros(size);
int r = exp > 0 ? ModuloPowerOfTwo.moduloPowerOfTwo(x, exp) : 0;

if (n <= 0) {
    throw new IllegalArgumentException("exponent n must be >= 1");
}
int r = ModuloPowerOfTwo.moduloPowerOfTwo(x, n);

static boolean validExponent(int n) { return n >= 1; }

try {
    int r = ModuloPowerOfTwo.moduloPowerOfTwo(x, n);
} catch (IllegalArgumentException e) {
    // n was <= 0; fall back to x mod 1 == 0 only if that is intended
}

When it happens

Trigger: Calling moduloPowerOfTwo(x, n) with n == 0 or n < 0. Common when n is computed as log2 of a size and the size is 1 (giving 0) or when an uninitialized/default int (0) is passed.

Common situations: Deriving the exponent from an array/buffer size via bit-length where the size is a power of two equal to 1; config value left at default 0; off-by-one in computing power from a divisor.

Related errors

• Input cannot be negative
• Input must be a non-empty binary string.
• Input must contain only '0' and '1'. Found: {}
• Input must contain only '0' and '1'.
• Invalid Baconian code: {}

AI-assisted analysis of TheAlgorithms/Java@fdfb9a395b (2026-08-13). Data as JSON: /api/errors/513b85a8e64190d7. Report an issue: GitHub.