TheAlgorithms/Java · error · IllegalArgumentException

Focal length and object distance must be non-zero.

Error message

Focal length and object distance must be non-zero.

What it means

Thrown by ThinLens.imageDistance(focalLength, objectDistance) when either focalLength or objectDistance is exactly 0.0. The thin-lens formula computes v = 1 / ((1/f) - (1/u)); a zero in either term makes 1/f or 1/u evaluate to Infinity, producing a meaningless (NaN/Infinity) image distance, so the guard rejects it up front.

Source

Thrown at src/main/java/com/thealgorithms/physics/ThinLens.java:38

 */
public final class ThinLens {

    private ThinLens() {
        throw new AssertionError("No instances.");
    }

    /**
     * Computes the image distance using the thin lens formula.
     *
     * @param focalLength focal length of the lens (f)
     * @param objectDistance object distance (u)
     * @return image distance (v)
     * @throws IllegalArgumentException if focal length or object distance is zero
     */
    public static double imageDistance(double focalLength, double objectDistance) {

        if (focalLength == 0 || objectDistance == 0) {
            throw new IllegalArgumentException("Focal length and object distance must be non-zero.");
        }

        return 1.0 / ((1.0 / focalLength) - (1.0 / objectDistance));
    }

    /**
     * Computes magnification of the image.
     *
     * <pre>
     *     m = v / u
     * </pre>
     *
     * @param imageDistance image distance (v)
     * @param objectDistance object distance (u)
     * @return magnification
     * @throws IllegalArgumentException if object distance is zero
     */
    public static double magnification(double imageDistance, double objectDistance) {

View on GitHub (pinned to fdfb9a395b)

Solutions

1. Initialize focalLength and objectDistance to a non-zero physical value (e.g. 10.0) before calling.
2. If the value comes from input, validate focalLength != 0 && objectDistance != 0 and surface a clear error before invoking imageDistance.
3. Guard against NaN/Infinity too if you accept computed values: Double.isFinite(focalLength) && Double.isFinite(objectDistance) && focalLength != 0 && objectDistance != 0.

Example fix

// before
ThinLens.imageDistance(focalLen, objDist); // focalLen or objDist may be 0.0

// after
if (Double.isFinite(focalLen) && Double.isFinite(objDist) && focalLen != 0.0 && objDist != 0.0) {
    double v = ThinLens.imageDistance(focalLen, objDist);
} else {
    throw new IllegalArgumentException("focalLen and objDist must be finite, non-zero doubles");
}

if (!Double.isFinite(focalLength) || !Double.isFinite(objectDistance)
        || focalLength == 0.0 || objectDistance == 0.0) {
    throw new IllegalArgumentException("focalLength and objectDistance must be finite and non-zero");
}
double v = ThinLens.imageDistance(focalLength, objectDistance);

// Java has no structural type narrowing; use a static guard method
static boolean validLensParams(double f, double u) {
    return Double.isFinite(f) && Double.isFinite(u) && f != 0.0 && u != 0.0;
}

try {
    double v = ThinLens.imageDistance(focalLength, objectDistance);
} catch (IllegalArgumentException e) {
    // log optics params at debug, degrade gracefully; do NOT retry with same values
    logger.warn("Invalid lens parameters: f={}, u={}", focalLength, objectDistance);
}

When it happens

Trigger: Call imageDistance(0.0, 10.0), imageDistance(5.0, 0.0), or imageDistance(0.0, 0.0). Any call where focalLength == 0 || objectDistance == 0 trips it. Note it does NOT reject negatives or NaN — only exact 0.0.

Common situations: Defaulting an optical parameter to 0 as an 'unset' sentinel; parsing a focal length from user input or a config file that defaulted to 0; physics simulations where a lens/object wasn't initialized before the first frame.

Related errors

• Object distance must be non-zero.
• Input '" + input + "' contains not only digits
• Orbiting mass and radius must be positive.
• Natural frequency must be positive.
• Damping coefficient must be non-negative.

AI-assisted analysis of TheAlgorithms/Java@fdfb9a395b (2026-08-13). Data as JSON: /api/errors/27971973ba2d0b6e. Report an issue: GitHub.