How to fix error 'Concurrent Modification Exception' from my code

Question

I tried running the for-loop, but it would not run since allWords was empty. Therefore I added the first if-statement, so allWorlds would have atleast one item, and therefor the loop would run. The loop now runs, but I get:

Exception in thread "main" java.util.ConcurrentModificationException
at java.util.ArrayList$Itr.checkForComodification(Unknown Source)
at java.util.ArrayList$Itr.next(Unknown Source)

A few google searches told me that this happens because I try to change the ArrayList while its in use (?). I wonder if there is an easy fix, or if I have to rewrite the code. Thanks in advance.

The code:

private void addWord(String word){

    if(allWords.isEmpty()){  //allWords defined above.
        allWords.add(word);
    }

    for (String check : allWords){

        if (!check.equalsIgnoreCase(word)){
            allWords.add(word);
        }
        else if(check.equalsIgnoreCase(word)){
            System.out.println("This word is a duplicate.");
        }
        else{
            System.out.println("Something went wrong.");
        }
    }
}

As suggested in the comments to your [earlier question](http://stackoverflow.com/questions/33109742/check-through-an-arraylist-if-it-contains-a-string-if-it-doesnt-add-that-stri), use a `Set` instead of a `List` if you do not want to allow duplicates in `allWords`. The return value of [`Set.add()`](https://docs.oracle.com/javase/7/docs/api/java/util/Set.html#add%28E%29) indicates whether the set already contained the value or not. — Mick Mnemonic, Oct 13 '15 at 21:46
Declaring `SortedSet allWords = new TreeSet(String.CASE_INSENSITIVE_ORDER);` will reduce your method into `if(!allWords.add(word)){System.out.println("This word is a duplicate.");}` — Mick Mnemonic, Oct 13 '15 at 21:50

score 1 · Answer 1 · answered Oct 13 '15 at 21:39

You're stepping through the items in your for loop - what happens when you add something to the list while you're doing that? Java doesn't know how to handle that, so it throws a ConcurrentModificationException exception.

The solution? Don't mess with the array while you're using it in the for loop.

One solution (and this is not the only one) is to create a second array to hold the items you want to add temporarily while you're running through the for loop, then go back and add those items when you're done.

However, I suspect you have an error in your logic in the code fragment above. The way it's written, it looks like word should be added multiple times. I think what you want is a boolean flag:

set a flag to false before you start your for loop
if you find word while you're iterating over your array of words, then set it to true and exit the for loop
once you're done with the for loop, check your flag; if it's false, then add your word (since you didn't find it in the array).

Also, the if...else if...else is redundant: equalsIgnoreCase can only return true or false, so there's no third alternative and the else will never be hit.

score 1 · Answer 2 · answered Oct 13 '15 at 21:40

Your ArrayList is in use because you are iterating over it. I would recomend you to make a second list for temporal data

so basically something like this:

List<String> tempList = new ArrayList<String>();
.....
if (!check.equalsIgnoreCase(word)){
    tempList.add(word);
}
.....

How to fix error 'Concurrent Modification Exception' from my code

2 Answers2