How to fix error 'Concurrent Modification Exception' from my code [duplicate]

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.");
        }
    }
}

Answer 1

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.

Answer 2

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);
}
.....