What is the difference between iterator.remove() ArrayList.remove()?

Question

Following code uses iterator.remove() ArrayList.remove().
public class CollectionRemove
    private static void checkIteratorRemove()
         List<String> list = new ArrayList<String>();     
          list.add("1");
          list.add("2");
          list.add("3");     
         System.out.println("in checkWithIterator*************");
        Iterator<String> iter = list.iterator();
        while (iter.hasNext()) {
            String str = iter.next();           
            if (str.equals("2")) {
                iter.remove();
            System.out.println("list Size: " + list.size() + " Element: " +  str);
    private static void checkListRemove()
         List<String> list = new ArrayList<String>();     
          list.add("1");
          list.add("2");
          list.add("3");     
        System.out.println("in ncheckWithForLoop*************");
        Iterator<String> iter = list.iterator();
        while (iter.hasNext()) 
             String str = (String) iter.next();    
            if (str.equals("2")) {
                list.remove(str);
            System.out.println("list Size: " + list.size() + " Element: " +  str);
    public static void main(String args[])
         checkIteratorRemove();
         checkListRemove();
output is :
in checkWithIterator*************
list Size: 3 Element: 1
list Size: 2 Element: 2
list Size: 2 Element: 3
in ncheckWithForLoop*************
list Size: 3 Element: 1
list Size: 2 Element: 2Why is this difference ? what is the difference between iterator.remove() ArrayList.remove()?

Answer 1

In the case of Fail-fast iterator, if a thread modifies a collection directly while iterating over it, the iterator will thow ConcurrentModificationException . Say,
for (Iterator it = collection.iterator(); it.hasNext()) {
    Object object = it.next();
    if (isConditionTrue) {
        // collection.remove(object);  can throw ConcurrentModificationException
        it.remove(object);
}As per specs,
Note that this exception does not always indicate that an object has been concurrently modified by a different thread. If a single thread issues a sequence of method invocations that violates the contract of an object, the object may throw this exception. For example, if a thread modifies a collection directly while it is iterating over the collection with a fail-fast iterator, the iterator will thow this exception.