iterator hasNext returns true but next throws exception

Question

I was just curious on how multiple iterators on a single object would behave and typed in the following code.

import java.util.Map;
import java.util.HashMap;
import java.util.Iterator;

class Test{
  public static void main(String[] a) {
    Map<Integer,String> map = new HashMap<>();
    map.put(1,"One");

    Iterator<Integer> it1 = map.keySet().iterator();
    Iterator<Integer> it2 = map.keySet().iterator();

    it1.next();
    it1.remove();
    System.out.println(it2.hasNext());
    System.out.println(it2.next());

    System.out.println(map.get(1));
  }
}

The map is empty as expected But, i thought Iterator#hasNext would return false , but instead it returned true and Iterator#next threw a ConcurrentModificationException .

when a value is removed from a Collection using the Iterator#remove method shouldn't the other iterator's hasNext method return false, as there wouldn't be any value to be returned?

Answer 1

The hasNext method only checks the next attribute:

public final boolean hasNext() {
  return next != null;
}

and the only method that modifies the next attribute are:

• the nextEntry method
• and the remove method

Which means that if you a few Iterator instances pointing to the same collection instance only when you call one of the above two methods the Iterator will access the underlying collection and check whether its state has change.

Please note that if the underlying collection is modified the behavior of an iterator is unspecified , which means it might be changed and you should not depend on it.

Answer 2

The javadoc for iterator.remove() states:

The behavior of an iterator is unspecified if the underlying collection is modified while the iteration is in progress in any way other than by calling this method.

And that is precisely what is happening in your code, the underlying collection is being modified for more than one iterator.