public class Block extends PhysicsElement  {
   private static int id=0;  // Block identification
   private final float mass;
   private float width;
   private double pos_t;
   private double pos_tPlusDelta;
   private double speed_t;
   private double speed_tPlusDelta;
   private Spring leftSpring, rightSpring;
   
   private Block(){   // nobody can create a block without state
     super(id++);
     mass=0;
   }
   public Block(float mass, float width, float position, float speed){
      super(id++);
      this.mass = mass;
      this.width = width;
      pos_t = position;
      this.speed_t = speed;
      leftSpring = rightSpring = null;
   }
   public Block(float mass, float width, float position, float speed, Spring lSpring, Spring rSpring){
     this(mass, width, position, speed);
     leftSpring = lSpring;
     rightSpring = rSpring;
   }
   public void attachLeftSpring (Spring lSpring) {
     leftSpring = lSpring;
   }
   public void attachRightSpring (Spring rSpring) {
     rightSpring = rSpring;
   }
   public double getPosition() {
      return pos_t;
   }
   public void computeNextState(double delta_t) {
     double force=0.0;
     if (rightSpring != null) force = rightSpring.getForce(this);
     if (leftSpring != null) force += leftSpring.getForce(this);
     double a = force/mass;
     pos_tPlusDelta = pos_t + speed_t*delta_t + a*delta_t*delta_t/2;
     speed_tPlusDelta = speed_t + a*delta_t;
   }
   public void updateState(){
     pos_t = pos_tPlusDelta;
     speed_t = speed_tPlusDelta;
   }
   public String getDescription() {
      return "Block_" + getId()+": x";
   }
   public String getState() {
      return getPosition()+"";
   }
}