Why is `ArrayList.add` causing a ConcurrentModificationException?

Question

Trying to wrap up a project. Everything works in it except it can't instantiate the animal into the designated array. This line is code that my instructor gave me, so I'm at even more of a loss. This is what I used to declare the array list:

private static ArrayList<Dog> dogList = new ArrayList<Dog>();

I then collect the information I needed and use the code below to try and add it to the array list. nthDog is a previously declared variable and the parameters are correct.

 Dog nthDog = new Dog(name, breed, gender, age, weight, acquisitionDate,
            acquisitionCountry, trainingStatus, reserved, inServiceCountry);
 dogList.add(nthDog);

This code was sent to me by my instructor, however, when used it pulls up the errors ConcurrentModificationException and checkForComodification. Can't figure out what could be wrong.

We need more code to see the cause for that sort of error, ideally to see all the places `dogList` is used, or more context around the `add` call. — Louis Wasserman, Aug 18 '21 at 18:26
random guess until then: are you adding to `dogList` while iterating over it? — Rogue, Aug 18 '21 at 18:29
`ConcurrentModificationException` requires there to be an `Iterator`, either explicitly or implicitly e.g. with a for-each loop or a copy constructor. Please can you show the code doing the iterating? — Andy Turner, Aug 18 '21 at 18:55

score 0 · Answer 1 · answered Aug 18 '21 at 18:37

You might be modifying a collection directly while it is iterating over the collection with a fail-fast iterator, the iterator will throw this exception.

For more details on ConcurrentModificationException Please refer https://docs.oracle.com/javase/7/docs/api/java/util/ConcurrentModificationException.html

Why is `ArrayList.add` causing a ConcurrentModificationException?

1 Answers1