Signatures of Point3D sub and mul does not match behavior of Point2D

Point2D and Point3D should have the same behavior in use and in method signatures. For example, Point2D's sub method does a minus-equals on the coordinates, whereas Point3D's sub method returns a new object of the subtracted result:

public void sub(Point2D other) { x -= other.x; y -= other.y; }
public Point3D sub(Point3D other) { return new Point3D(x - other.x, y - other.y, z - other.z); }

This could lead to mistakes when coding against the api.