Generic Observer - Casting problems..

Question

I'm trying to make a generic observer pattern, modeled after Wadler's book.
But I get type errors at the lines marked with ?? below, and although I fixed them, don't understand why the original code is wrong - any help please!
interface Observer<S extends Observable<S,O,A>,
                       O extends Observer  <S,O,A>,
                       A > {
     public void update ( S subject, A arg);
     // public void update ( Observable<S,O,A> subject, A arg);
abstract class Observable<S extends Observable<S,O,A>,
                             O extends Observer  <S,O,A>,
                             A > {
     List<Observer<S,O,A>> obs = new ArrayList<Observer<S,O,A>>();
     boolean changed = false;
    public void          notifyObservers(A a){
         // for ( O o : obs )            // ??
         //    o.update(this,a);          // ??
         for ( Observer<S,O,A> o : obs )
              o.update((S)this,a );     // Cast??
}

Answer 1

Stephan,
To illustrate the problem with your version and a client;
The type TestObsGen must implement the inherited abstract method Observer<Thing,TestObsGen,Integer>.update(Observable<Thing,TestObsGen,Integer>, Integer)
Any insights welcome!
public interface Observer<
     S extends Observable<S, O, A>,
     O extends Observer<S, O, A>,
     A> {
    public void update(Observable<S, O, A> subject, A arg);
abstract class Observable<
               S extends Observable<S, O, A>,
               O extends Observer<S, O, A>,
               A> {
    List<Observer<S, O, A>> obs = new ArrayList<Observer<S, O, A>>();
    boolean changed = false;
    public void notifyObservers(A a) {
        for (Observer<S, O, A> o : obs)
            o.update(this, a);
public class TestObsGen
     implements Observer< Thing, TestObsGen, Integer >
     Thing thing;
     TestObsGen ( Thing t ) {
          thing = t;
          // t.addObserver(this);
     void update(Thing t, Integer count) {
          System.out.println("Count:" + count);
     public static void main(String[] args) {
          Thing t = new Thing();
          TestObsGen test = new TestObsGen(t);
          t.bump();
          // Event received?!
class Thing
     extends Observable< Thing, TestObsGen, Integer >
     int count;
     void bump() {
          count++;
          // setChanged();
          // notifyObservers();
}