about ConcurrentModificationException?

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!