Concurrent Modification Exception : adding to an ArrayList

I normally use something like this:

for (Element element : new ArrayList<Element>(mElements)) {
    ...
}

quick, clean and bug-free

another option is to use CopyOnWriteArrayList

You're not allowed to add an entry to a collection while you're iterating over it.

One option is to create a new List<Element> for new entries while you're iterating over mElements, and then add all the new ones to mElement afterwards (mElements.addAll(newElements)). Of course, that means you won't have executed the loop body for those new elements - is that a problem?

At the same time, I'd recommend that you update your code to use the enhanced for loop:

for (Element element : mElements) {
    ...
}

An indexed for loop should also work.

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

Using Iterators also fixes concurrency problems, like this:

Iterator<Object> it = iterator.next().iterator();
while (it.hasNext()) {
    it.remove();
}

adding from list in this case leads to CME, no amount of synchronized will let you avoid that. Instead, consider adding using the iterator...

        for(ListIterator<Element> it = mElements.listIterator(); it.hasNext();){
            Element element = it.next();

            if(touchX > element.mX  && touchX < element.mX + element.mBitmap.getWidth() && touchY > element.mY   
                    && touchY < element.mY + element.mBitmap.getHeight()) {  

                //irrelevant stuff..

                if(element.cFlag){
                    // mElements.add(new Element("crack",getResources(), (int)touchX,(int)touchY));
                    it.add(new Element("crack",getResources(), (int)touchX,(int)touchY));
                    element.cFlag = false;

                }           
            }
        }

Also I think it's somewhat slippery to state like...

...The problem occurs at Element element = it.next();

for the sake of precision note that above is not guaranteed.

API documentation points out that this ...behavior cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast operations throw ConcurrentModificationException on a best-effort basis...

I solved creating a lock (Kotlin):

import java.util.concurrent.locks.ReentrantLock

Class A {
    private val listLock = ReentrantLock()
    fun doSomething(newElement){
        listLock.lock()
        list.add(newElement)
        listLock.unlock()
    }
}

You could use an auto-decrement for loop, and deal with the additional elements next time.

List additionalElements = new ArrayList();
for(int i = mElements.size() - 1; i > -1 ; i--){
    //your business
    additionalElements.add(newElement);
}
mElements.add(additionalElements);

Well i have tried all the aspects in my case where i was iterating in an adapter a list but due to hitting again and again i showed me the message of exception being thrown . I tried Casting the list to

 = (CopyOnWriteArraylist<MyClass>)mylist.value;

but it also throwed me an exception of CouldNotCastException,(and i finally pondered over the fact that why do they use or provide us a facality of casting).

I even used the so called Synchronized Block too, but even it didn't worked or i might would have been using it in a wrong way.

Thus it's all when i finally used the #all of time# Technique of handling the exception in try catch block, and it worked So put your code in the

try{
//block

}catch(ConcurrentModificationException){
//thus handling my code over here
}

The accepted solution (to create a copy of the collection) usually works well.

However, if the Element contains another collection this does not make a deep copy!

Example:

class Element {
   List<Kid> kids;

   getKids() {
      return kids;
   }
}

Now when you create a copy of the List of Elements:

for (Element element : new ArrayList<Element>(elements)) { ... }

You can still get a ConcurrentModificationException if you iterate over element.getKids() and, parally, alter the kids of that element.

Looking back it's obvious, but I ended up in this thread so maybe this hint helps others, too:

class Element {
   List<Kid> kids;

   getKids() {
      // Return a copy of the child collection
      return new ArrayList<Kid>(kids);
   }
}

    LinkedList<Double> newList = new LinkedList<>();
    newList.addAll(oldList);

This will take care of ConcurrentModificationException and good practice