[java] How to avoid "ConcurrentModificationException" while removing elements from `ArrayList` while iterating it?

I'm trying to remove some elements from an ArrayList while iterating it like this:

for (String str : myArrayList) {
    if (someCondition) {
        myArrayList.remove(str);
    }
}

Of course, I get a ConcurrentModificationException when trying to remove items from the list at the same time when iterating myArrayList. Is there some simple solution to solve this problem?

You can use the iterator remove() function to remove the object from underlying collection object. But in this case you can remove the same object and not any other object from the list.

from here

One alternative method is convert your List to array, iterate them and remove them directly from the List based on your logic.

List<String> myList = new ArrayList<String>(); // You can use either list or set

myList.add("abc");
myList.add("abcd");
myList.add("abcde");
myList.add("abcdef");
myList.add("abcdefg");

Object[] obj = myList.toArray();

for(Object o:obj)  {
    if(condition)
        myList.remove(o.toString());
}

While other suggested solutions work, If you really want the solution to be made thread safe you should replace ArrayList with CopyOnWriteArrayList

    //List<String> s = new ArrayList<>(); //Will throw exception
    List<String> s = new CopyOnWriteArrayList<>();
    s.add("B");
    Iterator<String> it = s.iterator();
    s.add("A");

    //Below removes only "B" from List
    while (it.hasNext()) {
        s.remove(it.next());
    }
    System.out.println(s);

List myArrayList  = Collections.synchronizedList(new ArrayList());

//add your elements  
 myArrayList.add();
 myArrayList.add();
 myArrayList.add();

synchronized(myArrayList) {
    Iterator i = myArrayList.iterator(); 
     while (i.hasNext()){
         Object  object = i.next();
     }
 }

No, no, NO!

In single threated tasks you don't need to use Iterator, moreover, CopyOnWriteArrayList (due to performance hit).

Solution is much simpler: try to use canonical for loop instead of for-each loop.

According to Java copyright owners (some years ago Sun, now Oracle) for-each loop guide, it uses iterator to walk through collection and just hides it to make code looks better. But, unfortunately as we can see, it produced more problems than profits, otherwise this topic would not arise.

For example, this code will lead to java.util.ConcurrentModificationException when entering next iteration on modified ArrayList:

        // process collection
        for (SomeClass currElement: testList) {

            SomeClass founDuplicate = findDuplicates(currElement);
            if (founDuplicate != null) {
                uniqueTestList.add(founDuplicate);
                testList.remove(testList.indexOf(currElement));
            }
        }

But following code works just fine:

    // process collection
    for (int i = 0; i < testList.size(); i++) {
        SomeClass currElement = testList.get(i);

        SomeClass founDuplicate = findDuplicates(currElement);
        if (founDuplicate != null) {
            uniqueTestList.add(founDuplicate);
            testList.remove(testList.indexOf(currElement));
            i--; //to avoid skipping of shifted element
        }
    }

So, try to use indexing approach for iterating over collections and avoid for-each loop, as they are not equivalent! For-each loop uses some internal iterators, which check collection modification and throw ConcurrentModificationException exception. To confirm this, take a closer look at the printed stack trace when using first example that I've posted:

Exception in thread "main" java.util.ConcurrentModificationException
    at java.util.AbstractList$Itr.checkForComodification(AbstractList.java:372)
    at java.util.AbstractList$Itr.next(AbstractList.java:343)
    at TestFail.main(TestFail.java:43)

For multithreading use corresponding multitask approaches (like synchronized keyword).

You have to use the iterator's remove() method, which means no enhanced for loop:

for (final Iterator iterator = myArrayList.iterator(); iterator.hasNext(); ) {
    iterator.next();
    if (someCondition) {
        iterator.remove();
    }
}

As an alternative to everyone else's answers I've always done something like this:

List<String> toRemove = new ArrayList<String>();
for (String str : myArrayList) {
    if (someCondition) {
        toRemove.add(str);
    }
}
myArrayList.removeAll(toRemove);

This will avoid you having to deal with the iterator directly, but requires another list. I've always preferred this route for whatever reason.

If you want to modify your List during traversal, then you need to use the Iterator. And then you can use iterator.remove() to remove the elements during traversal.

Java 8 user can do that: list.removeIf(...)

    List<String> list = new ArrayList<>(Arrays.asList("a", "b", "c"));
    list.removeIf(e -> (someCondition));

It will remove elements in the list, for which someCondition is satisfied