TheAlgorithms/Java · error · IllegalArgumentException

Length must be positive

Error message

Length must be positive

What it means

Thrown by the SimplePendulumRK4 constructor when length <= 0. The pendulum's angular frequency is omega = sqrt(g / L); a non-positive length makes this undefined or imaginary and is physically meaningless. This is the first guard, checked before the gravity guard in the same constructor.

Source

Thrown at src/main/java/com/thealgorithms/physics/SimplePendulumRK4.java:26

 */
public final class SimplePendulumRK4 {

    private SimplePendulumRK4() {
        throw new AssertionError("No instances.");
    }

    private final double length; // meters
    private final double g; // acceleration due to gravity (m/s^2)

    /**
     * Constructs a simple pendulum simulator.
     *
     * @param length the length of the pendulum in meters
     * @param g the acceleration due to gravity in m/s^2
     */
    public SimplePendulumRK4(double length, double g) {
        if (length <= 0) {
            throw new IllegalArgumentException("Length must be positive");
        }
        if (g <= 0) {
            throw new IllegalArgumentException("Gravity must be positive");
        }
        this.length = length;
        this.g = g;
    }

    /**
     * Computes the derivatives of the state vector.
     * State: [theta, omega] where theta is angle and omega is angular velocity.
     *
     * @param state the current state [theta, omega]
     * @return the derivatives [dtheta/dt, domega/dt]
     */
    private double[] derivatives(double[] state) {
        double theta = state[0];
        double omega = state[1];

View on GitHub (pinned to fdfb9a395b)

Solutions

1. Pass a strictly positive length in meters (e.g. 1.0 for a typical lab pendulum).
2. Validate length at the configuration boundary and reject zero/negative values upstream.
3. Guard NaN explicitly if length is computed: require !Double.isNaN(length) && length > 0.

Example fix

// before
SimplePendulumRK4 p = new SimplePendulumRK4(0, 9.81);
// after
SimplePendulumRK4 p = new SimplePendulumRK4(1.0, 9.81);

if (Double.isNaN(length) || length <= 0) {
    throw new IllegalArgumentException("length must be > 0");
}
SimplePendulumRK4 p = new SimplePendulumRK4(length, g);

When it happens

Trigger: Calling new SimplePendulumRK4(length, g) with length == 0, a negative length, or (commonly) a length field that was never initialised. Note: NaN length will NOT trip this guard (NaN <= 0 is false) and will surface later.

Common situations: Uninitialised length defaulting to 0, a unit-conversion error (cm passed where m expected and divided again), or a measurement of zero from a broken sensor.

Related errors

• Gravity must be positive
• Natural frequency must be positive.
• Damping coefficient must be non-negative.
• Orbiting mass and radius must be positive.
• State must be a non-null array of length 2.

AI-assisted analysis of TheAlgorithms/Java@fdfb9a395b (2026-08-13). Data as JSON: /api/errors/c3e3f72df1d853d5. Report an issue: GitHub.