Iterator and ListIterator

java.util.Iterator<T>

An iterator over a collection.

Iterator takes the place of Enumeration in the Java collections framework.

Iterators differ from enumerations in two ways:

1. Iterators allow the caller to remove elements from the underlying collection during the iteration with well-defined semantics.

2. Method names have been improved.

public interface ListIterator<E> extends Iterator<E>

An iterator for lists that allows the programmer to traverse the list in either direction, modify the list during iteration, and obtain the iterator's current position in the list.

A ListIterator has no current element; its cursor position always lies between the element that would be returned by a call to previous() and the element that would be returned by a call to next().

An iterator for a list of length n has n+1 possible cursor positions, as illustrated by the carets (^) below:

                 Element(0)   Element(1)   Element(2)   ... Element(n-1

cursor positions:^            ^            ^            ^                  ^

Note that the remove and set(Object) methods are not defined in terms of the cursor position; they are defined to operate on the last element returned by a call to next or previous().

package com.collection;

import java.util.ArrayList;

import java.util.Iterator;

import java.util.List;

import java.util.ListIterator;

public class IteratorVsListIterator {

       public static void main(String[] args) {

              List<String> list = new ArrayList<String>();

              list.add("rajesh");

              list.add("sattu");

              list.add("shan");

              System.out.println("iterator :");

              Iterator<String> iterator = list.iterator();

              while (iterator.hasNext()) {

                     System.out.println(iterator.next());

                    

              }

             

             

              ListIterator<String> listItr = list.listIterator();

             

              System.out.println("List Iterator : next");

              while (listItr.hasNext()) {

                     System.out.println(listItr.next());

                     listItr.add("rahul");

              }

             

              System.out.println("List Iterator : previous");

              while (listItr.hasPrevious()) {

                     System.out.println(listItr.previous()); 

              }

       }

}

Key points about Iterator:

Ø  Iterator in Java support generics so always use Generic version of Iterator rather than using Iterator with raw type.

Ø  If you want to remove objects from Collection than don't use for-each loop instead use Iterator's remove () method to avoid any ConcurrentModificationException.

Ø  Iterating over collection using Iterator is subject to ConcurrentModificationExc if Collection is modified after Iteration started, but this only happens in case of fail-fast Iterators.

Ø  There are two types of Iterators in Java, fail-fast (ArrayList, HashMap) and fail-safe (CopyOnWriteList, ConcurrentHashMap).

Ø  List collection type also supports ListIterator which has add() method to add elements in collection while Iterating.