public float angleBetween(Vector3f otherVector) {
        float dotProduct = dot(otherVector);
        float angle = FastMath.acos(dotProduct);
        return angle;
    }
Returns the angle (in radians) between this vector and the argument,
provided both vectors have length=1. The current instance is unaffected.

@param otherVector a unit vector to compare (not null, unaffected)
@return the angle (in radians, not negative)
jme3-core/src/main/java/com/jme3/math/Vector3f.java