Simple ArrayList.remove(int) doesn't work. What I am doing wrong?

Question

I am trying to iterate through an ArrayList and remove the actual object (index s) under certain conditions (x == 0). It always gives an error at the line where the object should be removed, if executed. Without the remove() it runs perfectly fine.

int s = 0;
int x = 0;
if (!objectList.isEmpty()) {
    for (obj actualObj : objectList) {
        if (x == 0) {
            objectList.remove(s);
        } else {
            System.out.println("x != 0");
        }
        s++;
    }
} else {
    System.out.println("list is empty");
}

You don't need to iterate to remove an element if you already know its index — Peter Hull, Jul 03 '20 at 13:45
https://stackoverflow.com/questions/18448671/how-to-avoid-concurrentmodificationexception-while-removing-elements-from-arr — Shadov, Jul 03 '20 at 13:45
What are `x` and `s` here? they seem to always be `0`, and aren't effected by the loop — Mureinik, Jul 03 '20 at 13:46
your code seems logicaly faulted, maybe you do not get the logic behind what you want to achieve, anyway see https://howtodoinjava.com/java/collections/arraylist/arraylist-remove-example/ — Nikos M., Jul 03 '20 at 13:47
Ok, thanks @all, solved my problem by simple pasting "break;" after the "remove()" AND replacing "s" by "actualObject". Works like a charm. For everyone who doesn't understand the logic: I didn't posted the whole code of 200 lines, only the part that is not working. — user11740260, Jul 03 '20 at 14:24

score 1 · Answer 1 · answered Jul 03 '20 at 13:54

My biggest nitpick would be this: you can't mutate/modify the collection you are iterating over (outside of calling Iterator#remove, etc) when using syntactic sugar like the enhanced for-each loop (for (T val : Iterable<T>)). In fact, Iterator#remove exists pretty much exactly for this reason:

int toRemove = 0; //The number to remove from your list
List<Integer> list = /* your list */;
Iterator<Integer> itr = list.iterator(); //Create an iterator of the collection
while (itr.hasNext()) { //while new elements are available...
    Integer val = itr.next(); //grab the next available element
    if (val == toRemove) { //removal condition
        itr.remove(); //remove the last grabbed element from the collection
    }
}

If you're capable of using Java 8:

int toRemove = 0; //The number to remove from your list
List<Integer> list = /* your list */;
list.removeIf(val -> toRemove == val); //didn't use member reference, for clarity reasons

Arvind Kumar Avinash · Answer 2 · 2020-07-03T13:56:42.183

Each time you remove an element from a List using an index, the List shrinks because the succeeding elements shift one place left/down. It means that in the next iteration of the loop, you will be actually deleting an element which was next to the next of the last deleted element.

For this reason, you should delete elements from a List in a loop using an Iterator instead of the index as shown below:

Iterator<YourType> itr = objectList.iterator();
while (itr.hasNext()) {
    YourType obj = itr.next();
    if (some condition) {
        itr.remove();
    }
    // ...
}

score 0 · Answer 3 · answered Jul 03 '20 at 13:53

you are removing from the list while iterating through it. A workable option is

    int x = 0;
    List<String> objectList = Arrays.asList("A", "B", "C");
    if (objectList.isEmpty()) System.out.println("list is empty");

    List<String> filtered = objectList.stream().filter(s1 -> x != 0).collect(Collectors.toList());
    System.out.println(objectList + "\n" + filtered);

Simple ArrayList.remove(int) doesn't work. What I am doing wrong?

3 Answers3