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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)
Olimpiu POP
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user2681668
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3 Answers3

7

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());
Subhrajyoti Majumder
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3

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());

}
Aniket Thakur
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0

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();
criszhao
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