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);
    }
}

Answer 1

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()

Answer 2

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);
    }
}