I'm trying to understand the Observer and the Observable.
Here's an example that I'm trying to figure out:
public class IntegerDataBag extends Observable implements Iterable<Integer> {
private ArrayList<Integer> list= new ArrayList<Integer>();
public void add(Integer i){
list.add(i);
setChanged();
notifyObservers();
}
public Iterator<Integer> iterator(){
return list.iterator();
}
public Integer remove (int index){
if (index< list.size()){
Integer i = list.remove(index);
setChanged();
notifyObservers();
return i;
}
return null;
}
}
public class IntegerAdder implements Observer {
private IntegerDataBag bag;
public IntegerAdder(IntegerDataBag bag) {
this.bag = bag;
bag.addObserver(this);
}
public void update(Observable o, Object arg) {
if (o == bag) {
System.out.println("The contents of the IntegerDataBag have changed");
}
}
}
The
bag.addObserver()
can be made only becauseIntegerDataBag
extendsObservable
?Where is this observer being add to? What is being created and where?
What is the difference between
setChanged()
andnotifyObservers()
?I don't understand the
update
method; what doesarg
stand for? Why do I need to check thato==bag
? Why would I update another observable?Why should I need this observer anyway?