Concurrent Modification Exception: Adding to an Arraylist

Concurrent Modification Exception : adding to an ArrayList

ConcurrentModificationException occurs when you modify the list (by adding or removing elements) while traversing a list with Iterator.

Try

List<Element> thingsToBeAdd = new ArrayList<Element>();
for(Iterator<Element> it = mElements.iterator(); it.hasNext();) {
    Element element = it.next();
    if(...) {  
        //irrelevant stuff..
        if(element.cFlag){
            // mElements.add(new Element("crack",getResources(), (int)touchX,(int)touchY));
            thingsToBeAdd.add(new Element("crack",getResources(), (int)touchX,(int)touchY));
            element.cFlag = false;
        }           
    }
}
mElements.addAll(thingsToBeAdd );

Also you should consider enhanced for each loop as Jon suggested.

java.util.ConcurrentModificationException thrown when adding to List

The ConcurrentModificationException is thrown when calling String value = it.next();. But the actual culprit is list.add("6");. You mustn't modify a Collection while iterating over it directly. You are using it.remove(); which is fine, but not list.add("6");.

While you can solve the problem with Streams, I will first offer a solution with Iterators, as this is a good first step for understanding the problem.

You need a ListIterator<String> if you want to add and remove during iteration:

for (ListIterator<String> it = list.listIterator(); it.hasNext();){
    String value = it.next();

    if (value.equals("4")) {
        it.remove();
        it.add("6");
    }

    System.out.println("List Value: " + value);
}

This should do the trick!

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A solution using Streams:

List<String> newList = list.stream()
        .map(s -> s.equals("4") ? "6" : s)
        .collect(Collectors.toList());

Here we create a Stream from your List. We map all values to themselves, only "4" gets mapped to "6" and then we collect it back into a List. But caution, newList is immutable!

Throwing ConcurrentModificationException if we add elements in ArrayList at the end while iterating

There is probably a better solution, but this one is working as well. A second list is used and then added to the original one, after the loop has finished.

Main

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

public class Main {

    public static void main(String[] args) {
        List<Integer> al = new ArrayList<Integer>();
        List<Integer> toAdd = new ArrayList<Integer>();
        int stepSize = 10;

        al.add(20);
        al.add(30);
        al.add(40);

        Iterator<Integer> it = al.iterator();
        while (it.hasNext()) {
            int value = it.next();
            if (value == al.get(al.size() - 1)){
                toAdd.add(value + stepSize);
            }
        }

        // Add all elements
        al.addAll(toAdd);

        // Print
        al.forEach(System.out::println);
    }
}

In case you only want to append one element based on the last value, as in your example, you could also wait until the loop is finished (if you still need it) and try to add it like this:

al.add(al.get(al.size() - 1) + stepSize);

Output

20
30
40
50

Java ConcurrentModificationException with an ArrayList

This code has multiple flaws. In this form it is going to add new Player instances until an entry with matching name is found. Starting an iteration, modifying a list, and continuing the iteration will throw a ConcurrentModificationException as you observed. Furthermore you compare strings with == instead of equals.

  for (Player p : inGamePlayers) {

    if (newUsername == p.username) { // This needs to be .equals
      System.out.println(p.username+" reconnected!");
      break;
    }
    // this line runs many times
    inGamePlayers.add(new Player(newUsername));
  }

Instead I suggest you to extract that code to a new function, and change the control flow:

private static void handleConnected(ArrayList<Player> inGamePlayers, String newUsername) {
  for (Player p : inGamePlayers) {
    if (newUsername.equals(p.username)) {
      System.out.println(p.username+" reconnected!");
      return; // return instead of break
    }
  }
  // we did not return, so this user is new
  inGamePlayers.add(new Player(newUsername));
}

// ...

public static void main(String[] args) {
  ArrayList<Player> inGamePlayers = new ArrayList<Player>();
  inGamePlayers.add(new Player("Griff"));

  String newUsername = "Polak";

  // Call this function in place of the old loop
  handleConnected(inGamePlayers, newUsername);

  for (Player p : inGamePlayers) {
    System.out.println(p.username);
  }
}

How to fix ConcurrentModificationException error when adding items during iteration

ListIterator has the add method, you can use it like this:

ListIterator<User> iterator = users.listIterator();
while(iterator.hasNext()) {
    User u = iterator.next();
    if(!user.getID().equals(u.getID())) {
        iterator.add(user);
    }
}

But to add a user only if it doesn't exist in the list you don't need an iterator. If you do it inside of the loop, you will add a user for each of the list's users that doesn't have the same id. But instead you need to check that the user's id is dirrenet from all users' ids and add the user only once.
If you are using Java 8, you could do something like this:

    boolean userExists = users.stream().filter(u -> u.getID().equals(user.getID())).findFirst().isPresent();
    if (!userExists) {
        users.add(user);
    }

ConcurrentModificationException while trying to delete an item from ArrayList

You cannot call remove() from inside a "for-each" loop (the for (item : collection) structure). Doing so will throw that ConcurrentModificationException.

If you want to loop and remove items while looping, you can use a traditional "for" loop:

for (int i = 0; i < itemStorico.size(); ++i) {
    ItemModel itemModel2 = itemStorico.get(i);
    if (...) {
        itemStorico.remove(i);
        ...
    }
}

However, this will introduce a subtle bug. Imagine you're looping over [a,b,c,d]. If you are at index 1 and remove b from the collection, everything will "slide over" and you'll have [a,c,d]. When you next look at index 2, you'll have skipped over c.

You can avoid the bug by manually decrementing the index when you remove, but it's a little gross.

...
    itemStorico.remove(i);
    --i;
...

You can also avoid the problem altogether by using the collection's iterator().

for (Iterator<ItemModel> iterator = itemStorico.iterator(); iterator.hasNext(); ) {
    ItemModel itemModel2 = iterator.next();
    if (...) {
        iterator.remove();
        ...
    }
}

ArrayList throwing `ConcurrentModificationException` when trying to run `.size()` method

The most glaring issue you have there is that you're using ArrayList.subList() while you don't seem to understand what it really does:

Returns a view of the portion of this list ... The returned list is backed by this list.

What you're storing in messagesInFlight is a view, not a copy. When you delete the first messages from messages, you're in fact deleting the very messages you had in your messagesInFlight right after subList() call. So after the for loop, there will be completely different messages, and the first n messages will be completely lost.

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As to why you are getting the error you see - subList() specifically allows non-structural changes to both sub-list and the original list (non-structural means replacing the elements, not adding or removing them), and the example in the documentation also showcases how the original list may be modified by modifying the sub-list. However, modifying the original list and then accessing it through the sub-list is not allowed and may result in ConcurrentModifcationException, similarly to what happens when you change a list you're iterating through with an iterator.

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