TheAlgorithms/Java · error · IllegalArgumentException

Invalid input parameters

Error message

Invalid input parameters

What it means

Thrown by MonteCarloIntegration.approximate/doApproximate when validate() fails. validate() requires ALL of: fx != null, a < b (strict — a == b is invalid), and n > 0. The single generic message hides which condition failed, so you must check all three.

Source

Thrown at src/main/java/com/thealgorithms/randomized/MonteCarloIntegration.java:65

    /**
     * Approximates the definite integral of a given function over a specified
     * interval using the Monte Carlo method with a random seed based on the
     * current system time for more randomness.
     *
     * @param fx    the function to integrate
     * @param a     the lower bound of the interval
     * @param b     the upper bound of the interval
     * @param n     the number of random samples to use
     * @return      the approximate value of the integral
     */
    public static double approximate(Function<Double, Double> fx, double a, double b, int n) {
        return doApproximate(fx, a, b, n, new Random(System.currentTimeMillis()));
    }

    private static double doApproximate(Function<Double, Double> fx, double a, double b, int n, Random generator) {
        if (!validate(fx, a, b, n)) {
            throw new IllegalArgumentException("Invalid input parameters");
        }
        double total = 0.0;
        double interval = b - a;
        int pairs = n / 2;
        for (int i = 0; i < pairs; i++) {
            double u = generator.nextDouble();
            double x1 = a + u * interval;
            double x2 = a + (1.0 - u) * interval;
            total += fx.apply(x1);
            total += fx.apply(x2);
        }
        if ((n & 1) == 1) {
            double x = a + generator.nextDouble() * interval;
            total += fx.apply(x);
        }
        return interval * total / n;
    }

View on GitHub (pinned to fdfb9a395b)

Solutions

1. Ensure the integrand fx is a non-null Function<Double,Double> before calling.
2. Pass bounds with a < b strictly; if your interval is [b, a] with b > a, swap them (or negate the result).
3. Pass a positive sample count n (e.g. 1000 or more for usable accuracy).

Example fix

// before
MonteCarloIntegration.approximate(fx, upper, lower, 0); // bounds reversed, n=0

// after
double lo = Math.min(upper, lower);
double hi = Math.max(upper, lower);
double result = (fx != null && hi > lo && samples > 0)
    ? MonteCarloIntegration.approximate(fx, lo, hi, samples)
    : Double.NaN;

if (fx == null || !(a < b) || n <= 0) {
    throw new IllegalArgumentException("fx must be non-null, a < b, n > 0");
}
double approx = MonteCarloIntegration.approximate(fx, a, b, n);

static boolean validMonteCarlo(Function<Double,Double> fx, double a, double b, int n) {
    return fx != null && a < b && n > 0;
}

try {
    double approx = MonteCarloIntegration.approximate(fx, a, b, n);
} catch (IllegalArgumentException e) {
    // message is generic; re-check fx/a/b/n yourself to report the real cause
    logger.warn("Monte Carlo rejected params: fx={}, a={}, b={}, n={}", fx != null, a, b, n);
}

When it happens

Trigger: approximate(null, 0, 1, 100) (null function); approximate(f, 2, 1, 100) (a >= b); approximate(f, 0, 1, 0) (n <= 0); approximate(f, 0, 0, 10) (degenerate zero-width interval).

Common situations: Passing bounds in the wrong order (upper as lower); defaulting sample count to 0 in a config; a null Function reference when the integrand wasn't set; flipping a/b when integrating a descending interval.

Related errors

• Sample size cannot exceed stream size.
• Input '" + input + "' contains not only digits
• Focal length and object distance must be non-zero.
• Object distance must be non-zero.
• Input array cannot be null or empty.

AI-assisted analysis of TheAlgorithms/Java@fdfb9a395b (2026-08-13). Data as JSON: /api/errors/617b2b100f93b7c9. Report an issue: GitHub.