观察者模式问题

这段代码错在哪里？
import java.util.Observable;


public class AObservable extends Observable {
public void test(){
String str = "yao";

setChanged();
notifyObservers(str);
}

}

import java.util.Observable;
import java.util.Observer;


public class AObserver implements Observer{

private AObservable aObservable = null;

public AObserver(){
this.aObservable = new AObservable();
this.aObservable.addObserver(this);
}

@Override
public void update(Observable o, Object arg) {
System.out.println(arg.toString());

}

}

public class ClientTest {

public static void main(String[] args) {

AObservable a = new AObservable();
a.test();

}

}