Concurrentmodificationexception When Adding Inside a Foreach Loop in Arraylist

ConcurrentModificationException when adding inside a foreach loop in ArrayList

Java Collection classes are fail-fast which means that if the
Collection will be changed while some thread is traversing over it
using iterator, the iterator.next() will throw a ConcurrentModificationException.

This situation can come in case of multithreaded as well as single
threaded environment. - www.javacodegeeks.com

You can't modify a List in a for/each loop, which is syntactic sugar around the Iterator as an implementation detail. You can only safely call .remove() when using the Iterator directly.

Note that Iterator.remove is the only safe way to modify a collection
during iteration; the behavior is unspecified if the underlying
collection is modified in any other way while the iteration is in
progress. - Java Collections
Tutorial

Calling .add() inside the for/each loop modifies the contents, and the Iterator that is used behind the scenes sees this and throws this exception.

A more subtle concern is the that the second way you list, the .size() is increasing every time you .add() so you will end up processing all the things you .add(), this could possibly cause an endless loop depending on what the input data is. I am not sure if this is what you desire.

Solution

I would create another ArrayList and .add() to it all the new things, and then after the loop, use .addAll() on the original ArrayList to combine the two lists together. This will make things explicit in what you are trying to do, that is unless your intention is process all the newly added things as you add them.

2014 Solution:

Always use Immutable collections classes and build new Immutable collection classes instead of trying to modify a single shared one. This is basically what my 2012 answer says but I wanted to make it more explicit.

Guava supports this very well, use ImmutableList.copyOf() to pass around data.

Use Iterables.filter() to filter out stuff into a new ImmutableList, no shared mutable state, means no concurrency problems!

How to fix ConcurrentModificationException when using foreach to iterate over a List?

The exception results from adding an element to the models List while iterating over it.

You have to change your logic. I suspect the logic of your inner loop is wrong anyway, and fixing it will also solve your problem. You probably want to search first if the List contains any element matching model and modify it if found, and only add a new instance to the List if you don't find a match (i.e. after the loop is over).

Your inner loop would look like this:

if (models.isEmpty()) {
    models.add(model);
} else {
    boolean found = false;
    for (ProjetVTO projetModel : models) {
        if ((projetModel.getAnnee() == model.getAnnee()) && (projetModel.getPriorite().equals(model.getPriorite()))) {
            projetModel.setNbillets(projetModel.getNbillets() + model.getNbillets());
            found = true;
            break;
        }
    }
    if (!found) {
        models.add(model);
    }
}

or simply (you can eliminate the outer condition):

boolean found = false;
for (ProjetVTO projetModel : models) {
    if ((projetModel.getAnnee() == model.getAnnee()) && (projetModel.getPriorite().equals(model.getPriorite()))) {
        projetModel.setNbillets(projetModel.getNbillets() + model.getNbillets());
        found = true;
        break;
    }
}
if (!found) {
    models.add(model);
}

ConcurrentModificationException in foreach loop

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.

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.

How to avoid java.util.ConcurrentModificationException when iterating through and removing elements from an ArrayList

Two options:

• Create a list of values you wish to remove, adding to that list within the loop, then call originalList.removeAll(valuesToRemove) at the end
• Use the remove() method on the iterator itself. Note that this means you can't use the enhanced for loop.

As an example of the second option, removing any strings with a length greater than 5 from a list:

List<String> list = new ArrayList<String>();
...
for (Iterator<String> iterator = list.iterator(); iterator.hasNext(); ) {
    String value = iterator.next();
    if (value.length() > 5) {
        iterator.remove();
    }
}

why concurrent modification on foreach method but not on for loop

ForEach loop won't loop directly on your collection. It uses the iterator of your collection behind. you can see the iterator in your collections implementation.

From Arraylist source code

735 
736     public Iterator<E> More ...iterator() {
737         return new Itr();
738     }

An optimized version of AbstractList.Itr
742 
743     private class More ...Itr implements Iterator<E> {
744         int cursor;       // index of next element to return

And your foreach loop equals to

for(Iterator<Integer> i = targets.iterator(); i.hasNext(); ) {
  Integer element = i.next();
  //To do
}

So if you doing any operation here, and the same time you modifying the collection, iterator under the hood got confuse and throws exception.

From JLS

List<? extends Integer> l = ...
for (float i : l) ...

will be translated to:

for (Iterator<Integer> #i = l.iterator(); #i.hasNext(); ) {
    float #i0 = (Integer)#i.next();
    ...

ConcurrentModificationException in foreach loop even though I don't remove anything from arraylist

 records.add(record);

You modified the ArrayList this line.

You can turn your for-each loop to indexed-for loop to prevent ConcurrentModificationException, or maybe you can just create a new ArrayList to hold your records:

ArrayList<Record> records = helper.get();
ArrayList<Record> result = new ArrayList<Record>();
for (Record x : records) {
    Record record = new Record(many parameters);
    result.add(record);
}

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