Remove multiple items from ArrayList during iterator

Question

Is it safe to remove multiple items from an ArrayList while iterating through it with an iterator?

Iterator<String> iterator = nameList.iterator();
 while(iterator.hasNext()){
     String s = iterator.next();
     List<String> list = work(s);
     for (String s1 : list) {
        nameList.remove(s1);   
    }
}

The work() method gives back a list of names that should be removed from the nameList, during the runtime of while loop.

Decontrol

Answer 1

No, it's not safe, and can throw ConcurrentModificationException. You can collect all the elements to be removed in a temporary List, and then call list.removeAll(tmpList) after the while loop to perform the removal.

Iterator<String> iterator = nameList.iterator();
List<String> removed = new ArrayList<>();
while(iterator.hasNext()){
    String s = iterator.next();
    removed.addAll(work(s));
}
list.removeAll(removed);

I realize this can be less efficient, since you might be calling work(s) on Strings the should have been removed from the List earlier. This can be improved by changing tempList to a Set, and only calling work(s) for Strings not in the Set:

Iterator<String> iterator = nameList.iterator();
Set<String> removed = new HashSet<>();
while(iterator.hasNext()){
    String s = iterator.next();
    if (!removed.contains(s)) {
        removed.addAll(work(s));
    }
}
list.removeAll(removed);

Spelter

Yes, avoiding to call work(s) on a String that should have been removed in a previous loop is exactly what I want to do. – Decontrol

I will try your idea using a "removed" set. Thanks for the suggestion. – Decontrol

Of course, then you don’t need to deal with an Iterator manually; for(String s: nameList) { if(!removed.contains(s)) removed.addAll(work(s)); } list.removeAll(removed); – Tuttle

Answer 2

You can implement your logic if you use a ListIterator which is fail-safe. Below is a basic example :

Set<String> removed = new HashSet<>();
    ArrayList<String> nameList = new ArrayList<String>();

            ListIterator<String> iterator = nameList.listIterator();
             while(iterator.hasNext()){
                 String s = iterator.next();
                 if (!removed.contains(s)) {
        removed.addAll(work(s));
    }

            }


 nameList.removeAll(removed);
                 System.out.println(nameList);

With your logic, you have to account the performance. If performance is not a factor, you can go ahead and add/remove from a list through ListIterator.

Arbitrage

That still throws a ConcurrentModificationException – Decontrol

just check with sysout statements that the list you are getting is empty or anything. – Arbitrage

@Arbitrage calling remove() on the ListIterator exactly once per iteration step would be ok, but calling removeAll on the backing List is not. – Idiocrasy

Hi @Thommy, may be check whether you are removing an element from namelist which is already been removed or something...because my post is just to give you an idea of making use of ListIterator. – Arbitrage

@Arbitrage Yes that is exactly the problem, removing one works but I want to remove multiple in one iteration. I solved it using the "workaround" from Eran's answer. – Decontrol

Answer 3

As a previous answer said, removing an item from a iterated list is not safe. Here is a code that throws ConcurrentModificationException:

List<String> list = new ArrayList<>(Arrays.asList("1", "2", "3"));
Iterator<String> iterator = list.iterator();
while (iterator.hasNext()) {
    String s = iterator.next();
    list.remove("1");
}

Lowland

Answer 4

You can use Iterator.remove() (as opposed to nameList.remove(s)).

Iterator<String> iterator = nameList.iterator();
while(iterator.hasNext()){
    String s = iterator.next();
    iterator.remove();
}

Also consider nameList.clear() and nameList.removeAll(elementsToRemove).

Iterators are generally invalid after any modification to their underlying collections, except via the iterator itself. - When I sort a List what happens to its iterators?

https://docs.oracle.com/javase/8/docs/api/java/util/Iterator.html

Womanize