ConcurrentModificationException while removing duplicates in collection

Asked Nov 27 '17 at 10:06

Active Nov 27 '17 at 10:16

Viewed 40 times

I have a function to remove the duplicates in a collection.

/** Remove the duplicates **/
private List<Topic> removeDuplicateEntries(List<Topic> mlist) {

    List<Topic> allEvents = mlist;
    List<Topic> noRepeat = new ArrayList<Topic>();

    for (Topic event : allEvents) {
          boolean isFound = false;
          // check if the event name exists in noRepeat
          for (Topic e : noRepeat) {
                if (e.getUserId().equals(event.getUserId())){
                    isFound = true;
                    break;
                }
           }
           if (!isFound) noRepeat.add(event);
    }

    return noRepeat;
}

Whats happening is sometimes I get the error

java.util.ConcurrentModificationException
   at java.util.ArrayList$ArrayListIterator.next(ArrayList.java:573)
   at com.sttarter.communicator.CommunicatorDBHelper$1.run(CommunicatorDBHelper.java:54)
   at java.lang.Thread.run(Thread.java:818)

edited Nov 27 '17 at 10:16

savepopulation

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asked Nov 27 '17 at 10:06

Devrath

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4

use an `Iterator` instead of classic for loop – Mustapha Belmokhtar Nov 27 '17 at 10:07
dont use for each loop, use fori(classic for(int i=0;i – Manoj kumar Nov 27 '17 at 10:09
1

Neither will help if another thread changes the list concurrently. The code is fine as it is except for synchronization. – ewramner Nov 27 '17 at 10:10
1

there is another thread that is accessing and modifying `mlist` – Lino Nov 27 '17 at 10:10
1

why not `Set noRepeat = new HashSet<>(mlist);` – Youcef LAIDANI Nov 27 '17 at 10:10
1

Don't use a loop. Add `mlist` to a `Set` to remove duplicates. `Set result = new HashSet<>(mlst) `. – Henrik Aasted Sørensen Nov 27 '17 at 10:10
1

There's nothing inherently wrong with your code, but I think that the `List` you're given as a parameter is being modified in another thread somewhere. Consider making a copy of it before iterating through it. – Henrik Aasted Sørensen Nov 27 '17 at 10:17

ConcurrentModificationException while removing duplicates in collection

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