Java Observer Pattern - How to remove observers during update(notify) loop/iteration?

Question

I am very new to java so sorry in advance if anything I say sounds newbish, be gentle.

I have implemented a basic Observer Pattern. Some observers should only listen to one update and then immediately remove themselves from the observers/listeners list. However, whenever I tried doing that I got the famous java.util.concurrentmodificationexception error.

I'm obviously getting this error because I'm changing the list while still iterating over it, yet I am still unsure what is the right solution. I'm wondering if I'm even doing this the right way. If I am, what would be the needed fix to make it work? And if I'm not, I'd like to get suggestions for a better way of achieving what I'm trying to do.

Here's my code:

public interface Listener {
    public void onValueChange(double newValue);
}   


public class Observed {
    private int value;
    List<Listener>  listeners  = new ArrayList<>();

    public void addListener(Listener toAdd) {
        listeners.add(toAdd);
    }

    public void removeListener(Listener toRemove) {
        listeners.remove(toRemove);
    }

    public void changeValue(double newValue) {
        value = newValue;
        for (Listener l : listeners) l.onValueChange(newValue);                               
    }
}


public class SomeClassA implements Listener{
    private Observed observed;

    SomeClassA(Observed observed) {
        this.observed = observed;
    }

    @Override
    public void onValueChange(double newValue) {
        System.out.println(newValue);
        observed.removeListener(this);
    }
}


public class SomeClassB implements Listener{
   @Override
    public void onValueChange(double newValue) {
        System.out.println(newValue);
    } 
}



public class ObserverTest {
    public static void main(String[] args) {
        Observed observed = new Observed();
        SomeClassA objectA = new SomeClassA(observed);
        SomeClassB objectB = new SomeClassB();

        observed.addListener(objectB);
        observed.addListener(objectA);

        observed.changeValue(4);
    }
}

Missing cast before `value = (int) double;` – herry Oct 05 '13 at 12:14 — herry, Oct 05 '13 at 12:14

M Sach · Accepted Answer · 2013-10-05T12:22:44.127

7

one ways is to go fo CopyOnWriteArraylist instead of ArrayList .

CopyOnWriteArraylist is a thread-safe variant of ArrayList in which all mutative operations (add, set, and so on) are implemented by making a fresh copy of the underlying array.

Reason why its thrown in your case

you are modifying a collection directly while it is iterating over the collection under method changeValue()

edited Oct 05 '13 at 12:22

answered Oct 05 '13 at 12:08

M Sach

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score 1 · Answer 2 · answered Oct 05 '13 at 12:28

You can not remove items from a collection while you are iterating over it. That is, unless you use the Iterator#remove method. Since that is not a possibility in this case, an alternative is make a copy of your listener list and iterate over that instead. In that case the original listener list is free to be manipulated by the individual listeners:

public void changeValue(double newValue) {
    value = newValue;
    List<Listener> copyOfListeners = new ArrayList<Listener>(listeners);
    for(Listener l : copyOfListeners) {
        l.onValueChange(newValue);
    }
}

Instead of making a copy on each iteration, it is probably cheaper to make a copy on each modification of the list (which is usually more rare), and fortunately, there is already the `CopyOnWriteArrayList` from M Sach's answer that does that for you without further effort. — Philipp Wendler, Oct 05 '13 at 12:40

score 0 · Answer 3 · answered Oct 05 '13 at 12:43

the code below works, so you can try whatever it does.

import java.util.Observable;
import java.util.Observer;
class Model extends Observable {
    public void setX(double x) {
        this.x=x;
        System.out.println("setting x to "+x);
        setChanged();
        notifyObservers();
    }
    double x;
}
class A implements Observer {
    A(Model model) {
        this.model=model;
    }
    @Override public void update(Observable arg0,Object arg1) {
        System.out.println(getClass().getName()+" "+((Model)arg0).x);
        ((Model)arg0).deleteObserver(this);
    }
    Model model;
}
class B implements Observer {
    @Override public void update(Observable arg0,Object arg1) {
        System.out.println(getClass().getName()+" "+((Model)arg0).x);
    }
}
public class So19197579 {
    public static void main(String[] arguments) {
        Model model=new Model();
        model.addObserver(new A(model));
        model.addObserver(new B());
        model.setX(4);
        model.setX(8);
    }
}

Java Observer Pattern - How to remove observers during update(notify) loop/iteration?

3 Answers3