TheAlgorithms/Java · error · IllegalArgumentException

State must be array of length 2

Error message

State must be array of length 2

What it means

Thrown by SimplePendulumRK4.stepRK4 when the state array is null or its length is not exactly 2. The RK4 step expects a state vector [theta, omega] (angle, angular velocity) to compute four intermediate slopes; any other shape would cause index-out-of-bounds or null-dereference. This is the first guard, checked before the dt guard.

Source

Thrown at src/main/java/com/thealgorithms/physics/SimplePendulumRK4.java:59

     */
    private double[] derivatives(double[] state) {
        double theta = state[0];
        double omega = state[1];
        double dtheta = omega;
        double domega = -(g / length) * Math.sin(theta);
        return new double[] {dtheta, domega};
    }

    /**
     * Performs one time step using the RK4 method.
     *
     * @param state the current state [theta, omega]
     * @param dt the time step size
     * @return the new state after time dt
     */
    public double[] stepRK4(double[] state, double dt) {
        if (state == null || state.length != 2) {
            throw new IllegalArgumentException("State must be array of length 2");
        }
        if (dt <= 0) {
            throw new IllegalArgumentException("Time step must be positive");
        }

        double[] k1 = derivatives(state);
        double[] s2 = new double[] {state[0] + 0.5 * dt * k1[0], state[1] + 0.5 * dt * k1[1]};

        double[] k2 = derivatives(s2);
        double[] s3 = new double[] {state[0] + 0.5 * dt * k2[0], state[1] + 0.5 * dt * k2[1]};

        double[] k3 = derivatives(s3);
        double[] s4 = new double[] {state[0] + dt * k3[0], state[1] + dt * k3[1]};

        double[] k4 = derivatives(s4);

        double thetaNext = state[0] + dt / 6.0 * (k1[0] + 2 * k2[0] + 2 * k3[0] + k4[0]);
        double omegaNext = state[1] + dt / 6.0 * (k1[1] + 2 * k2[1] + 2 * k3[1] + k4[1]);

View on GitHub (pinned to fdfb9a395b)

Solutions

1. Always pass a 2-element double[] of the form [theta, omega], e.g. new double[]{0.1, 0.0} for a small initial angle at rest.
2. If you carry a longer state elsewhere, copy the [theta, omega] slice into a length-2 array before stepping.
3. Initialize the state at simulation start rather than reusing a stale reference.

Example fix

// before
double[] state = new double[]{0.1};
double[] next = p.stepRK4(state, 0.01);
// after
double[] state = new double[]{0.1, 0.0};
double[] next = p.stepRK4(state, 0.01);

double[] state = (s == null || s.length != 2) ? new double[]{0.0, 0.0} : s;
double[] next = p.stepRK4(state, dt);

static boolean isValidPendulumState(double[] s) {
    return s != null && s.length == 2;
}

When it happens

Trigger: Calling stepRK4(null, dt), stepRK4(new double[]{theta}, dt) (length 1), or stepRK4 with a state array that came from a different integrator using a longer layout.

Common situations: Mixing state arrays between the pendulum integrator and another physics module, a null state left over after a failed init, or a copy/truncate that dropped an element.

Related errors

• State must be a non-null array of length 2.
• Time step must be positive
• Time step must be positive.
• Orbiting mass and radius must be positive.
• Natural frequency must be positive.

AI-assisted analysis of TheAlgorithms/Java@fdfb9a395b (2026-08-13). Data as JSON: /api/errors/84faa2633fe2593f. Report an issue: GitHub.