Description Adrian Colyer 2005-09-07 02:53:36 EDT

(From aspectj-users)

The program below produces the erroneous message:

    [iajc] D:\workdir\DPsample\main\src\com\designpattern\observer\ObserverProt
ocol.aj:39 [error] Type mismatch: cannot convert from Collection to Collection
    [iajc] return observers;
    [iajc]        ^^^^^

//Subject.java
interface Subject {
 public void addObserver(Observer observer);
 public void removeObserver(Observer observer);
 public Collection getObservers();
}
//Observer.java
interface Observer
{
 public void update();
}

//ObserverProtocol
public abstract aspect ObserverProtocol{
 abstract pointcut stateChange(Subject subject);
 after(Subject subject):stateChange(subject){
   Iterator it=subject.getObservers().iterator();
   while(it.hasNext()){
        Observer observer=(Observer)it.next();
        observer.update();
   }
 }
 private Collection Subject.observers=new ArrayList();
 public void Subject.addObserver(Observer observer){
        observers.add(observer);
 }
 public void Subject.removeObserver(Observer observer){
       observers.remove(observer);
 }
 public Collection Subject.getObservers()
 {
  return observers;
 }
}