public abstract class PhysicsElement {
   private final int myId; /* to identify each element within its category */
   
   protected PhysicsElement( int id){
      myId = id;
   }
   protected int getId() {
      return myId;
   }
   public abstract String 	getDescription();
   public abstract void 	computeNextState(double delta_t, MyWorld world);
   public abstract void 	updateState();
   public abstract void		printState();
}