ConcurrentModificationException

While iterating over the List, or anything which implements iteratable, we cannot modify the contents of that particular collection. Not a surprise, but today while implementing iteratable (@ work) I din't know how to achieve this. BTW, why do I need to throw this exception ?? do I care ?? should I not just use object locking ?? I guess all of us know the reason, object state in multithreaded applications & performance. Anyways, I think below implementation looks decent enough:

<br />/**<br /> * Bag<br /> * @author pardeep<br /> */<br />public class Bag<E> implements Iterable<E> {<br /><br /> List<E> elements = null;<br /> int modifications = 0;<br /> <br /> public Bag(){<br /> elements = new ArrayList<E>();<br /> }<br /><br /> public Iterator<E> iterator(){<br /> return new Itr();<br /> }<br /> public int size(){<br /> return elements.size();<br /> }<br /> public void add(E element){<br /> elements.add(element);<br /> modifications ++;<br /> }<br /> public void remove(E element){<br /> elements.remove(element);<br /> modifications ++;<br /> }<br /> public void removeAll(){<br /> for (int i = 0; i < elements.size() ; i++)<br /> elements.remove(i);<br /> }<br /><br /> private class Itr implements Iterator<E>{<br /> int curr = 0;<br /> int expectedMod = modifications;<br /> public void remove(){<br /> // lets not implements this one right now.<br /> throw new UnsupportedOperationException();<br /> }<br /><br /> public boolean hasNext(){<br /> return curr < size();<br /> }<br /><br /> public E next(){<br /> checkForModification();<br /> return elements.get(curr++);<br /> }<br /> private void checkForModification(){<br /> if (expectedMod != modifications)<br /> throw new ConcurrentModificationException();<br /> }<br /> }<br />}<br />