Java ConcurrentModificationException when using list.remove()

Question

I've got a method called removeSup which is supposed to remove an object Supplement from a list of supplements. this is the code for the method:

private static void removeSup(Supplement supToRemove, List<Supplement> listToRemoveFrom) {
   Iterator<Supplement> iterator = listToRemoveFrom.iterator();
                while(iterator.hasNext()){
                    if(iterator.next().equals(supToRemove)){
                        iterator.remove();
                    }
                }
}

there is a class called magazine which defines the list of supplements.

public class Magazine {
  private List<Supplement> supList;
  public List<Supplement> getSupList() {
        return this.supList;
    }
  public void setSupList(List<Supplement> supList) {


      this.supList = supList;
        }
public Magazine(Double cost, String _name){
        this.supList = new ArrayList<>();
        this.weekCost = cost;
        this.name = _name;
    }
    }

the class supplement has the following constructor

public Supplement(String _name, Double _price, String _magName ){
        this.name=_name;
        this.price=_price;
        this.magName = _magName;
    }

in the main class client there is a search that the user can do to remove a certain Supplement

private static void searchSup(){
   System.out.println("Search for Supplement");
        String search = scanner.nextLine();
        for (Supplement sup : magazine.getSupList()) {
            if (!sup.getSupName().equalsIgnoreCase(search)) {
         //do something
        }
        else{
              removeSup(sup,magazine.getSupList());
        }
    }

} the main method in the client class is as follows:

 private Magazine magazine;
        public static void main(String[] args) {
                magazine = new Magazine(3.0, "pop");
                List<Supplement> startList = new ArrayList<>();
            startList.add(new Supplement("Nat Geo", 3.0,"pop"));
            startList.add(new Supplement("Discovery", 5.0,"pop"));
            startList.add(new Supplement("Health", 6.3,"pop"));
            startList.add(new Supplement("IT", 8.3,"pop"));
            magazine.setSupList(startList);
            searchSup();
        }

When I run this program and type any of the added supplements, i get an error

Exception in thread "main" java.util.ConcurrentModificationException
    at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:859)
    at java.util.ArrayList$Itr.next(ArrayList.java:831)
    at Client.searchSup(Client.java:131)
    at Client.searchSup(Client.java:140)
    at Client.main(Client.java:588)

is it the for loop i am using to search giving me an error? if so how would i go about fixing this?

did you read the javadoc for ConcurrentModificationException? also, did you search SO for similar problems? — jtahlborn
– jtahlborn, Commented Apr 3, 2014 at 1:06
Instead of traversing the list again in removeSup with a new iterator, iterate with an explicit iterator in searchSup and use that iterator's remove in searchSup. — user2357112
– user2357112, Commented Apr 3, 2014 at 1:07
possible duplicate of ConcurrentModificationException and a HashMap — jtahlborn
– jtahlborn, Commented Apr 3, 2014 at 1:08
@user2357112 i tried doing that and getting the same error. `else{ magazine.getSupList().remove(sup); } like that right? — Pindo
– Pindo, Commented Apr 3, 2014 at 1:39

rpmartz · Accepted Answer · 2014-04-03 01:06:55Z

0

You generally shouldn't modify a Collection while iterating over it. It's fine to modify elements, but you really shouldn't remove something from a Collection while iterating. See here: http://docs.oracle.com/javase/7/docs/api/java/util/ArrayList.html. Also, the Javadoc for ConcurrentModificationException may be helpful.

You might try returning a new list with the Supplement removed:

 private static List<Supplement> removeSup(Supplement supToRemove, List<Supplement> listToRemoveFrom) {
    List<Supplement> filteredSupplements = new ArrayList<Supplement>();
    for(Supplement supplement : listToRemoveFrom) {
       if(!suppplement.equals(supToRemove)){
           filteredSupplements.add(supplement);
       }

    }

    return filteredSupplements;
}

answered Apr 3, 2014 at 1:06

rpmartz

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2 Comments

Pindo Over a year ago

thanks. is there any way i can go around removing an element from the list?

rpmartz Over a year ago

The Collection interface has a remove() method but you'll need to read the Javadoc for each collection that implements it.

jacktan · Accepted Answer · 2014-04-03 01:26:38Z

0

It seams that the "magazine" is local var in the method of main, not accessible to searchSup.Fix it like private void searchSup(Magazine magazine) { //... } and more details if you can provide, the codes in Line 131 and 140 will be helpful.

answered Apr 3, 2014 at 1:26

jacktan

1

Comments

Pindo · Accepted Answer · 2014-04-03 02:18:21Z

0

I figured out that the search i was doing was not working with what i wanted to do so i created a method which returns an integer of the Supplement in the list.

private static int indexOfSup(List<Supplement> supSearchList, String nameOfSup) {
        for (Supplement sup : supSearchList) {
            if (sup.getSupName().equalsIgnoreCase(nameOfSup)) {
                return supSearchList.indexOf(sup);
            }
        }
        return -1;
    }

i then use this integer to remove from the list. a simple List.Remove(index) worked fine

Thanks for all the replies.

answered Apr 3, 2014 at 2:18

Pindo

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1 Comment

rpmartz Over a year ago

You're really doing a lot of work that Java could be doing for you. Look up the Collection and List interfaces, specifically their remove() methods as well as how to override equals() for an object.

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