java.util.ConcurrentModificationException while looping through arrayList

Question

Following is my code piece:

ArrayList<Contact> contactList = presenter.getContactList();
for (Contact contact : contactList) {
    if (null != contact) {
        String name = contact.getVehicleNumber();
        final ArrayList<Vehicle> vehicleList = contact.getVehicles();
        if (vehicleList != null && vehicleList.size() > 0) {
            name = vehicleList.get(0).getVehicleNumber();
        }
    }
}

I am getting java.util.ConcurrentModificationException. I have gone through other answers for similar questions but I am not doing any modifications to the original list by adding or removing items. Strange thing is it doesn't happen always. Will this Exception occur if presenter.getContactList() is modified by some other thread while this code piece is executed ?

Stack trace,

12-23 12:08:35.961 24843 25128 E AndroidRuntime: java.util.ConcurrentModificationException
12-23 12:08:35.961 24843 25128 E AndroidRuntime:        at java.util.AbstractList$SimpleListIterator.next(AbstractList.java:62)

`vehicleList.at(0)` generates a compiler error on my machine. Also, what line generates the `ConcurrentModificationException`? — cameron1024, Jan 28 '20 at 08:32
```vehicleList.at(0)``` should be ```vehicleList.get(0)``` which I wrote by mistake while copying from my project. The ```Exception``` happens on ```line 2``` on the for loop line. — Spring Breaker, Jan 28 '20 at 08:38
"For example, it is not generally permissible for one thread to modify a Collection while another thread is iterating over it" - from javadocs. So yes, it can't be modified by some other thread. — Amongalen, Jan 28 '20 at 08:38
How it works `vehicleList.length()`? Should use `vehicleList.size()` instead — Md. Asaduzzaman, Jan 28 '20 at 08:48
Try using `ArrayList contactList = new ArrayList<>(presenter.getContactList());` instead — Md. Asaduzzaman, Jan 28 '20 at 08:58
Have you used an iterator instead of the for loop and was able to reproduce this error — papaya, Jan 28 '20 at 08:59
@SiddarthSreeni Since I am not modifying the list, I didn't used it. How it will be useful anyway ? — Spring Breaker, Jan 28 '20 at 09:02
Is this an actual copy paste of your code? Are you using multiple threads? Can you add the full stacktrace? — Wesley De Keirsmaeker, Jan 28 '20 at 09:02
@WesleyDeKeirsmaeker No it's not. I modified the original code to make it presentable. The code pieces are from a SDK so can't share it. Yes multiple threads are used. — Spring Breaker, Jan 28 '20 at 09:04
Multiple threads are accessing the same data structures? Are you expecting it wont? — papaya, Jan 28 '20 at 09:12
Can you try by making a deep copy instead of a shallow copy? I think its making a shallow copy and that is the reason why this problem is originating. — Pronoy999, Jan 28 '20 at 10:06
If you are using multiple threads, check out concurrent lists: https://stackoverflow.com/questions/6916385/is-there-a-concurrent-list-in-javas-jdk . It's possible that multiple threads try to access the arrayList at the same time. — Wesley De Keirsmaeker, Jan 28 '20 at 11:48

java.util.ConcurrentModificationException while looping through arrayList

0 Answers0