public float smallestAngleBetween(Vector2f otherVector) {
        float dotProduct = dot(otherVector);
        float angle = FastMath.acos(dotProduct);
        return angle;
    }
Returns the unsigned angle between the current instance and the argument,
provided both vectors have length=1. If {@code otherVector} is null, Pi/2
is returned. The current instance is unaffected.

@param otherVector the unit vector to compare (unaffected) or null for
    none
@return the angle in radians (not negative)
jme3-core/src/main/java/com/jme3/math/Vector2f.java