Concurrent Modification Exception In ArrayList

Hi All,

My code is like this,

for(Iterator iterator = list.iterator(); iterator.hasNext();) { String string = iterator.next().toString(); list.add(string + "Hello"); iterator.remove(); }

Its a simple program. I've list with some names as values like,

Ram, Sham, Bham, Bhim, Suraj.

Now, I want to add "Hello" to each name by running above code. While iterating, first time I get Ram, I want to add "Hello" to Ram and add to list as "Ram Hello" also I want to remove original Value i.e "Ram". When I run above code it is giving Concurrent Modification Exception. How to do this?

Thanks:
Ramakrishna K.C

I think you need to add the modified strings to a second list and replace the original list's content (or the target of the reference) with this second list.

As you can't directly manipulate the value of the objects due to immutability of String, I'd say your best option is to use the set() method of a ListIterator:
List<String> list = new ArrayList<String>(); // ... for (ListIterator<String> iterator = list.listIterator(); iterator.hasNext();) { iterator.set(iterator.next()+ "Hello"); }

Ramakrishna Udupa wrote:
My code is like this,

for(Iterator iterator = list.iterator(); iterator.hasNext();) { String string = iterator.next().toString(); list.add(string + "Hello"); iterator.remove(); }

Basically, you are not allowed to change the list (without using the iterator) while you are iterating. If you want to change the list while iterating, you need to do it with the iterator (and not with the list).

Ramakrishna Udupa wrote:
Its a simple program. I've list with some names as values like,

Ram, Sham, Bham, Bhim, Suraj.

Now, I want to add "Hello" to each name by running above code. While iterating, first time I get Ram, I want to add "Hello" to Ram and add to list as "Ram Hello" also I want to remove original Value i.e "Ram". When I run above code it is giving Concurrent Modification Exception. How to do this?

Instead of using the add() and remove() methods, perhaps using the set() method (which does replacement) is a better option. Also, it would be a good idea to use a java.util.ListIterator instead, as that iterator as the methods that you need.

[EDIT: and I got beaten to the answer by three minutes]

Henry

If you want to replace "Suraj" with "Hello Suraj", you should not use add. Nor should you use an Iterator. You should iterate the List with an ordinary for loop and use the set method. As you will find here, that technique works well with arrayed lists, but may be slow for a linked list.

Campbell Ritchie wrote:Nor should you use an Iterator...

Hmmm. Not so sure I agree with that (providing, as the others said, you use a ListIterator), because it will highlight any concurrent update.
Also, the technique you describe is likely to be much slower for linked lists, whereas using an iterator for an array-based List will likely only be marginally slower than using a loop.

Winston

I never realised ListIterator has a set() method. You are right Winston, but I was beaten to the post twice within 4 minutes