Re: about ConcurrentModificationException?

From:

Daniel Pitts <newsgroup.spamfilter@virtualinfinity.net>

Newsgroups:

comp.lang.java.help

Date:

Sat, 01 Dec 2007 18:47:50 -0800

Message-ID:

<noadnVje281Cgc_anZ2dnUVZ_qCunZ2d@wavecable.com>

rmn190 wrote:

import java.util.*;

public class TryIteratorRemove {
    public static void main(String [] args){
        Collection<String> myCollection = new ArrayList<String>(10);

        myCollection.add("123");
        myCollection.add("456");
        myCollection.add("789");

        int i=0;

        for(Iterator it = myCollection.iterator();it.hasNext();) {
            String myObject = (String)it.next();
            System.out.println(myObject);

            i++;

            if(i==1){
                //myCollection.remove(myObject);
                it.remove();
            }
        }

        System.out.println("After remove,the size of myCollection is: " +
                       myCollection.size()+" \n and its content is: ");

        for(String s : myCollection){
            System.out.println(s);
        }
    }
}

    For the code above,why there is an exception
"ConcurrentModificationException" thrown when i want to remove the
String of "123" from myCollection,using the statement of
"myCollection.remove(myObject);"?and using the statement of
"it.remove()",there is not.

    Please help with that.

    Thanks a lot in advance!

Because, iterators keep track of how many changes are made through
themselves, and how many are made elsewhere. If the numbers don't add
up, that means that the iterator might not be valid.

Think of it this way, you have an ArrayList...
You iterate a copy of times, and them someone removes the first element.
The iterator would need to know that its index into the array had
changed by one, but there is no easy way to inform the iterator. The
array list would either have to keep track of ALL the iterators ever
created, or all of the changes that ever happened to it. Either way,
you would run out of memory very quickly.

HTH,
Daniel.

--
Daniel Pitts' Tech Blog: <http://virtualinfinity.net/wordpress/>