java.util.ConcurrentModificationException while inserting in ArrayList [duplicate]

It happens due to array list is modified after creation of Iterator.

The iterators returned by this ArrayList's iterator and listIterator methods are fail-fast: if the list is structurally modified at any time after the iterator is created, in any way except through the iterator's own remove or add methods, the iterator will throw a ConcurrentModificationException. Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future.

Documentation

Iterator<String> lir = al.iterator(); // Iterator created

while (lir.hasNext()) 
    System.out.println(lir.next());
al.add(2, "inserted"); // List is modified here
while (lir.hasNext()) 
    System.out.println(lir.next());// Again it try to access list 

What you should do here create new iterator object after modification.

...
al.add(2, "inserted");
lir = al.iterator();
while (lir.hasNext()) 
    System.out.println(lir.next());

You are modifying the Collection and then trying to use the same iterator.

1. Get the Collection iterator again

   al.add(2, "inserted");
   Iterator<String> lirNew = al.iterator();
   while (lirNew.hasNext()) {
   System.out.println(lirNew.next());
   }
   

2. or Use ListIterator

   ArrayList<String> al = new ArrayList<String>();
   
   al.add("S1");
   al.add("S2");
   al.add("S3");
   al.add("S4");
   
   ListIterator<String> lir = al.listIterator();
   
   while (lir.hasNext()) {
       System.out.println(lir.next());
   
   }
   
   lir.add("insert");
   
   while (lir.hasNext()) {
       System.out.println(lir.next());
   
   }
   

You add object to list, after the iterator is instanced. This will change the value of modCount in inner class AbstractList$Itr.class. next() method of iterator will call checkForComodification() method, which throws a ConcurrentModificationException. And this is so called fail-fast.

 //add in abstractList
 public void add(int index, E element) {
    if (index<0 || index>size)
        throw new IndexOutOfBoundsException();
    checkForComodification();
    l.add(index+offset, element);
    expectedModCount = l.modCount;
    size++;
    modCount++;  //modCount changed
}

In AbstractList$Itr

int expectedModCount;

public E next() {
        checkForComodification(); // cause ConcurrentModificationException
    try {
    E next = get(cursor);
    lastRet = cursor++;
    return next;
    } catch (IndexOutOfBoundsException e) {
    checkForComodification();
    throw new NoSuchElementException();
    }
}

 private void checkForComodification() {
    if (l.modCount != expectedModCount)  //modCount not equals to itr.expectedModCount
        throw new ConcurrentModificationException();
}

redo this code after your add:

al.add(2, "inserted");
lir = al.iterator();