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

Question

import java.util.ArrayList;
import java.util.Iterator;
import java.util.ListIterator;

public class MyList {
    public static void main(String[] args) {
        ArrayList<String> al = new ArrayList<String>();

        al.add("S1");
        al.add("S2");
        al.add("S3");
        al.add("S4");

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

        while (lir.hasNext()) {
            System.out.println(lir.next());
        }

        al.add(2, "inserted");

        while (lir.hasNext()) {
           System.out.println(lir.next());
        }
    }
}

The particular piece of code throws an error:

Exception in thread "main" java.util.ConcurrentModificationException
    at java.util.ArrayList$Itr.checkForComodification(Unknown Source)
    at java.util.ArrayList$Itr.next(Unknown Source)
    at collections.MyList.main(MyList.java:32)

Answer 1

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());

Answer 2

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());
   
   }