public class Spring extends PhysicsElement  {
   private static int id=0;  // Spring identification
   private float restLength;
   private final float stiffness;
   private Block leftBlock, rightBlock;

   private Spring(){  // nobody can create a block without state
     super(id++);
     stiffness=0;
   }
   public Spring(float restLength, float stiffness){
     super(id++);
     this.restLength = restLength;
     this.stiffness = stiffness;
     leftBlock = rightBlock = null;
   }
   public Spring(float restLength, float stiffness, Block lBlock, Block rBlock){
     this(restLength, stiffness);
     leftBlock = lBlock;
     rightBlock = rBlock;
   }
   public void attachLeftBlock (Block lBlock) {
     leftBlock = lBlock;
   }
   public void attachRightBlock (Block rBlock) {
     rightBlock = rBlock;
   }
   public double getLength() {
     if (rightBlock == null || leftBlock == null) return restLength;
      return (rightBlock.getPosition() - leftBlock.getPosition());
   }
   public double getForce(Block block) {
     double force = (getLength()-restLength)*stiffness;
     if (block == leftBlock) return force;
     if (block == rightBlock) return (-1)*force;
     else return 0.0;
   }
   public void computeNextState(double delta_t) {
   }
   public void updateState(){
   }
   public String getDescription() {
      return "Spring_"+ getId()+": x0,x1";
   }
   public String getState() {
      String s;
      if (leftBlock != null) 
         s = leftBlock.getPosition()+",";
      else if (rightBlock != null)
              s = (rightBlock.getPosition()-restLength)+",";
            else  
              s = "0,";  // up to here left end is determined 
      if (rightBlock != null)
         s += rightBlock.getPosition();
      else if (leftBlock != null)
               s += (leftBlock.getPosition()+restLength);
            else
               s += "0"; 
      return s;
   }
}