import java.awt.*;

public class Spring extends PhysicsElement {
   private static int id=0;  // Spring identification
   protected final double restLength;
   private final double stiffness;
   protected Attachable a_end, b_end;
   protected SpringView view;

   private Spring(){   // nobody can create a spring without state
      this(0,0);
   }
   public Spring(double restLength, double stiffness){
      super(id++);
      this.restLength = restLength;
      this.stiffness = stiffness;
      a_end = b_end = null;
      view = new SpringView(this);
   }
   public void attachEnd (Attachable sa) {  // note: we attach a spring to a ball, 
      if(a_end==null)                             //       not the other way around.
         a_end = sa;                     
     // to be completed by you
   }
   private Vector2D getStrech() {   // if you feel that this method in useful  
   }
   public Vector2D getForce(Ball ball) {
     // to be completed by you
   }
   public Vector2D getAendPosition() {
     return a_end.getPosition();
   }
   public Vector2D getBendPosition() {
     return b_end.getPosition();
   }
   public double getRestLength() {
     return restLength;
   }
   public void paintView (Graphics2D g) {   // NEW
     view.paint(g);  // update this spring's view in Model-View-Controller design pattern     
   }

}