TheAlgorithms/Java · error · IllegalArgumentException

Alpha must be between 0 and 1.

Error message

Alpha must be between 0 and 1.

What it means

Thrown by the EMAFilter constructor when the smoothing factor alpha is outside the valid half-open interval (0, 1]. An exponential moving average requires a positive weight on the current sample (alpha > 0), and a weight above 1 would amplify rather than smooth the signal. The boundary alpha = 0 (no update) and alpha > 1 (divergent filter) are both meaningless for an EMA and are rejected.

Source

Thrown at src/main/java/com/thealgorithms/audiofilters/EMAFilter.java:27

 * The smoothing factor (alpha) controls the degree of smoothing.
 *
 * <p>
 * Based on the definition from
 * <a href="https://en.wikipedia.org/wiki/Moving_average">Wikipedia link</a>.
 */
public class EMAFilter {
    private final double alpha;
    private double emaValue;

    /**
     * Constructs an EMA filter with a given smoothing factor.
     *
     * @param alpha Smoothing factor (0 < alpha <= 1)
     * @throws IllegalArgumentException if alpha is not in (0, 1]
     */
    public EMAFilter(double alpha) {
        if (alpha <= 0 || alpha > 1) {
            throw new IllegalArgumentException("Alpha must be between 0 and 1.");
        }
        this.alpha = alpha;
        this.emaValue = 0.0;
    }

    /**
     * Applies the EMA filter to an audio signal array.
     * EMA formula:
     * EMA = alpha * currentSample + (1 - alpha) * previousEMA
     *
     * @param audioSignal Array of audio samples to process
     * @return Array of processed (smoothed) samples
     */
    public double[] apply(double[] audioSignal) {
        if (audioSignal == null || audioSignal.length == 0) {
            return new double[0];
        }
        double[] emaSignal = new double[audioSignal.length];

View on GitHub (pinned to fdfb9a395b)

Solutions

1. Ensure alpha is a fraction strictly greater than 0 and at most 1: clamp or validate before constructing, e.g. require 0 < alpha <= 1.
2. If alpha comes from a period N, derive it as alpha = 2.0 / (N + 1.0) and reject N < 1 upstream.
3. If the value originates from user/CLI input, parse and bounds-check it before passing it to the constructor.

Example fix

// before
new EMAFilter(0);   // throws

// after
if (alpha <= 0 || alpha > 1) throw new IllegalArgumentException("alpha must be in (0,1]");
new EMAFilter(alpha);

public static boolean isValidAlpha(double alpha) {
    return alpha > 0.0 && alpha <= 1.0;
}
// usage
if (!isValidAlpha(alpha)) throw new IllegalArgumentException("alpha must be in (0,1]");
EMAFilter filter = new EMAFilter(alpha);

public static boolean isValidAlpha(double alpha) {
    return Double.isFinite(alpha) && alpha > 0.0 && alpha <= 1.0;
}

try {
    filter = new EMAFilter(alpha);
} catch (IllegalArgumentException e) {
    // fall back to a safe default smoothing factor
    filter = new EMAFilter(0.2);
}

When it happens

Trigger: Constructing `new EMAFilter(0.0)`, `new EMAFilter(-0.5)`, or `new EMAFilter(1.5)`. The check is `alpha <= 0 || alpha > 1`, so exactly 0 and anything strictly greater than 1 throw, while exactly 1.0 is accepted (pass-through).

Common situations: Reading alpha from a config file or CLI argument that defaults to 0 or is unset; computing alpha as `2/(N+1)` where N is derived from user input that can be 0 or negative; passing a percentage (e.g. 25 for 25%) instead of the fraction 0.25.

Related errors

• order must be greater than zero
• aCoeffs must be of size {}, got {}
• aCoeffs.get(0) must not be zero
• bCoeffs must be of size {}, got {}
• Invalid input: 0 ≤ k ≤ n is required.

AI-assisted analysis of TheAlgorithms/Java@fdfb9a395b (2026-08-13). Data as JSON: /api/errors/0c8ba101fee67f7c. Report an issue: GitHub.