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javacollectionsarraylistconcurrentmodification

ConcurrentModificationException in foreach loop

In my code:

    Collection<String> c = new ArrayList<>();
    Iterator<String> it = c.iterator();
    c.add("Hello");
    System.out.println(it.next());

Exception occures, because my collection changed after iterator created.

But what about in this code:

 ArrayList<Integer> list = new ArrayList<Integer>();
    list.add(1);
    list.add(2);
    list.add(3);
    for (Integer integer : list) {     // Exception is here
        if (integer.equals(2)) {
            list.remove(integer);
        }
    }

Why exception occured?

In second code, i did changes in my collection before for-each loop.

Solution

  • In the second loop, it's the same reason - you are removing an element from the list.

    To remove elements from a List while looping through it, either use standard old-fashioned for loops:

    for(int i=0;i<list.size();i++) {

    and remove list items inside that loop or use a ListIterator to iterate over the list.