List Iterator error - please explain

[Ronald Vermeij]

import java.util.*;
  public class ListExample {
  public static void main(String[] args) {
    // declaration
    List list = new ArrayList();
    Iterator elements = list.iterator();
    // als some elements to the list
    list.add("one");
    list.add("second");
    list.add("3rd");
    list.add(new Integer(4));
    list.add(new Float(5.0F));
    list.add("second");
    // duplicate element, is added to list
    list.add(new Integer(4)); // duplicate, is added

    //  Print contents of the list
    while (elements.hasNext()) {
      System.out.println(elements.next());
    }  // while
  } // main
} // class

When i compile this example i get the following error:
Exception in thread "main" java.util.ConcurentModificationException
at java.util.AbstractList$Itr.checkForComodification(AbstractList.java 372)
at java.util.AbstractList$Itr.next(AbstractList.java 343)
at ListExample.main(ListExample.java:18)

I want to print all the single contents of the list, controlled, element by element,
I want to do in in a different way then

 System.out.println(list);

question
Can you explain what i am doing wrong in understanding or coding?

Thansk for your hint,tips and tricks.
Ronald

[Ulf Dittmer]

That's because the list has been changed after you obtained the iterator. If you get the iterator after adding all the elements to the list (in other words, if you move line 6 after line 15), then it should work just fine.

[Amit Vinod Dali]

Just define the "Iterator" after adding elements and before using it to the list e.g.
just before

  while (elements.hasNext()) {  

ConcurrentModificationException

[Nicola Garofalo]

At line 6 you get an Iterator from an empty list, and then you begin filling the list with content.

Move line 6 to line 16.

And please have a look at the Runtime Exception ConcurrentModificationException

[Monika Joshi]

Initialise the iterator after adding the elements to list and not before you have added the elements to list.

Regards,
Minakshi

[Ulf Dittmer]

Oh boy, 4 people in total agreement!

[Campbell Ritchie]

Ulf Dittmer wrote:Oh boy, 4 people in total agreement!

It would be even more remarkable if rob and I had posted and agreed with each other

[Ronald Vermeij]

Thanks Ulf, Amit, Nicola && Minakshi for the explanation. I see where it went wrong!
1. In my mind i imagined a picture of a
- ArrayList as being literally an ARRAY of elements, things, stuff
-> [array_name||element1][[element2][....][[elementN]

- Iterator as being the "pointer" to an element of that same ARRAY
-> [array_name||element1][[element2][....][[elementN]
Iterator................^

Thanks to your explanation I now understand that:
- An Iterator - only should be created, getted AFTER a Collection-type, List,Set, has been filled with [elements].
- An Iterator - 'works the same as an "ArrayIndex pointer from C", walking from the Head to the Tail of the array.

Thanks everyone !

p.s
This "mind-mixup" comes from also having knowledge in other languages like Pascal, C, C++.
That is what makes me stumble most of the times... old thought and design patterns :-)
Time for a Brain.flush() operation on old language skills before adding more Java to it