// technically not required as vectors have the same information as points, but it's useful to have separate vector and point things..

public class Vector3D {
    public double x;
    public double y;
    public double z;

    public Vector3D(double _x, double _y, double _z){
        x = _x;
        y = _y;
        z = _z;
    }
    public double angleTo(Vector3D vec2){
        return Math.acos(
                (x*vec2.x + y*vec2.y + z*vec2.z)
                        /( //-----------------------------------------------------------
                        Math.sqrt(Math.pow(x, 2) + Math.pow(y, 2) + Math.pow(z, 2)) *
                        Math.sqrt(Math.pow(vec2.x,  2) + Math.pow(vec2.y, 2) + Math.pow(vec2.z, 2))));
    }
}