Question

The following code uses a simple list and displays elements contained in the list through a foreach loop and again through a while loop.

final public class Main
{
    public static void main(String... args)
    {           
        List<String>list=new ArrayList<String>();

        list.add("A");
        list.add("B");
        list.add("C");
        list.add("D");

        for(Object o:list)
        {
            System.out.printf("\n"+o);
        }

        Iterator<String>listIterator=list.iterator();

        //list.add("E");   If uncommented, throws an exception.
        //list.add("F");

        while(listIterator.hasNext())
        {
            System.out.printf("\n"+listIterator.next());
        }
    }
}

---

When the commented two lines above the while loop are uncommented, it throws an exception java.util.ConcurrentModificationException. If those commented lines are placed above this line Iterator<String>listIterator=list.iterator();, it works well. Such exceptions are very common while handling entities in EJBs. Why does this happen? How can it be avoided?

Answer 1

http://download.oracle.com/javase/6/docs/api/java/util/ArrayList.html

The iterators returned by this class’s iterator and listIterator methods are fail-fast: if the list is structurally modified at any time after the iterator is created, in any way except through the iterator’s own remove or add methods, the iterator will throw a ConcurrentModificationException.

Instead of using:

Iterator<String>listIterator=list.iterator();

use:

ListIterator<String> listIterator = list.listIterator();

This iterator provides an add() method for you to use. Note in the JavaDoc how it works though as it may not provide exactly what you need – it doesn’t add to the end of the list but rather a location relative to the ListIterator‘s position.