change a list while iterating or using a for each loop

Question

I am trying to iterate (or use a for each loop) on a Linked list class and be able to change the item (when found) to a passed in parameter.

for(Item n : items) 
{
    if (n.getKey().equals(key))
    {
         n = new Item(key, value);
    }
}

Does this change of data work or is it temporary (only to be lost when the activation record is deleted)?

Answer 1

You can't iterate over a collection and modify it. You will always get a java.util.ConcurrentModificationException. First off all you need to use an iterator, to remove the item. Then you can use a second list to store the data you want to add.

Here you are an example:

LinkedList<String> linkedList = new LinkedList<String>();

        linkedList.add("This");
        linkedList.add("is");
        linkedList.add("an");
        linkedList.add("test");

        LinkedList<String> temp = new LinkedList<String>();

        for (Iterator<String> iterator = linkedList.iterator(); iterator.hasNext();) {
            String string = (String) iterator.next();

            if(string.equals("an")) {
                iterator.remove();

                temp.add("a");
            }
        }

        linkedList.addAll(temp);

You can call iterator.remove() to savely remove the current item from list.

Answer 2

You are using fast enumeration, which protects the list that you are iterating through. If you would like to change the data in the list, you would need to use a traditional for loop.

Basically how fast enumeration works is it makes the array read-only in the block of code because you have no access to what integer the iteration is.

You could do this:

for(int i = 0; i < items.length; i++)
{
    if (n.getKey().equals(key))
    {
         items[i] = new Item(key, value);
    }
}