Move element of arraylist to last position

Question

I have an arrayList of complex objects Box. I need a specific element of these Boxes to be moved to the last position of the arrayList and have the original one removed like shown in the diagram:

My code is something like this, but I get this error: ConcurrentModificationException

for (Box i : boxes) {  //where boxes is the arrayList
 if (i.mouseOver()) {  //does the swapping I need if the mouse is over the box
  
  Box copy=i;
  boxes.remove(i);  //these 3 lines are where I think the mistake is
  boxes.add(copy);
 }
}

The problem seems to be that you are trying to modify a `Collection` on which you are iterating. You have to create a new `Collection` and modify that. — Tanbir Ahmed, Dec 07 '21 at 15:07
Does this answer your question? [How to avoid "ConcurrentModificationException" while removing elements from \`ArrayList\` while iterating it?](https://stackoverflow.com/questions/18448671/how-to-avoid-concurrentmodificationexception-while-removing-elements-from-arr) — Christopher Schneider, Dec 07 '21 at 15:08
Never use a *for-each* loop to modify a Java list! I suggest you to look at [this request](https://stackoverflow.com/questions/223918/iterating-through-a-collection-avoiding-concurrentmodificationexception-when-re), which explains your problem and their solutions — 0009laH, Dec 07 '21 at 15:09

John Bollinger · Accepted Answer · 2021-12-07T15:12:49.673

1

Once you modify the boxes (by removing and or adding elements) you cannot continue iterating over it. One way you can avoid that would be by breaking out of the loop once you move the Box you want to move:

for (Box i : boxes) {  //where boxes is the arrayList
    if (i.mouseOver()) {  //does the swapping I need if the mouse is over the box
        // No need to create a new reference
        boxes.remove(i);
        boxes.add(i);
        break;
    }
}

edited Dec 07 '21 at 15:12

answered Dec 07 '21 at 15:09

John Bollinger

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I think this would create `ConcurrentModificationException` – Turtle Dec 07 '21 at 15:10
Then you are wrong, @Turtle. Try it and see. The CME comes from attempting to continue the iteration after the list is modified (which this avoids, but the OP's does not), not from the modifications themselves. – John Bollinger Dec 07 '21 at 15:12
Okay, saw that... – Turtle Dec 07 '21 at 15:15

score 0 · Answer 2 · 2021-12-07T15:15:08.793

0

You cannot remove entries while in a for-each loop.

for (int i = 0; i < boxes.size(); i++) {
    if (boxes.get(i).mouseOver())
        boxes.add(boxes.remove(i));
}

edited Dec 07 '21 at 15:15

answered Dec 07 '21 at 15:08

This will be a never-ending `loop` – Turtle Dec 07 '21 at 15:21
@Turtle Nope, I tested it, and it works just fine. – Dec 07 '21 at 15:34

Rameshkanna Yohalingam · Answer 3 · 2021-12-07T15:19:00.903

If java.util.ConcurrentModificationException is the error you were getting, try not to use advanced for loop and just use regular for loop. Also in your code I see you are trying to remove the element from the list with the actual object. In Arraylist you can only remove by the index, not the element.

I think below should work.

for (int i = 0; i < boxes.size(); i++) {
    if (boxes.get(i).mouseOver()) { 
        boxes.add(boxes.remove(i));
    }
}

Move element of arraylist to last position

3 Answers3