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We already know that we can cycle over the elements of any collection class by calling iterator() method. For collection classes that implement List interface, we can iterate over their elements by not only iterator() method but also listIterator() method. listIterator() gives us an object that implements ListIterator interface.

Important features of ListIterator

• ListIterator works only in the access of a collection that has implemented List interface such as ArrayList, LinkedList etc. We cannot use ListIterator in Set, Queue, Deque.
• ListIterator allows a bidirectional access of a list, i.e. accessing a list in the forward or backward direction.
• ListIterator allows us add new elements to the list.
• ListIterator also lets us to modify the existing elements while iterating them.

Methods in ListIterator interface

Methods	Description
void add(E obj)	Inserts an object in the front of the list.
boolean hasNext()	Returns true if there is a next element in a list, otherwise, returns false.
E next()	Returns the next element in a list.
boolean hasPrevious()	Returns true if there is a previous element in a list, otherwise, returns false.
E previous()	Returns the previous element in a list.
void remove()	Removes the current element from the list.
void set(E obj)	Sets the value of the current element to value of obj.

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Using ListIterator to iterate over a collection

In this code, we are cycyling over the elements of a generic ArrayList<String>, in both directions (forward and backward) by -

• First calling listIterator() of ArrayList, which returns a listIterator.
• Using this ListIterator, we access its methods to acess the list bidirectionally.

import java.util.*;

class ListIteratorDemo
{
public static void main(String... ar)
{
ArrayList arr= new ArrayList();

arr.add("F");
arr.add("O");
arr.add("R");
arr.add("W");
arr.add("A");
arr.add("R");
arr.add("D");
ListIterator itr = arr.listIterator();

System.out.println("Iteraring the list in forward direction");
System.out.print("Contents of ArrayList are : ");
while(itr.hasNext())
{
System.out.print(itr.next() + " " );
}


System.out.println();

System.out.println("Iteraring the list in backward direction");
System.out.print("Contents of ArrayList are : ");

while(itr.hasPrevious())
{
System.out.print(itr.previous() + " " );
}

System.out.println();
System.out.println("Size of arraylist = "+ arr.size());

}
}

Output is :

Iteraring the list in forward direction
Contents of ArrayList are : F O R W A R D
Iteraring the list in backward direction
Contents of ArrayList are : D R A W R O F
Size of arraylist = 6

Program Analysis

• listIterator() method in ArrayList returns a ListIterator, pointing at the start of the ArrayList.
• Using ListIteator, we have cycled over the elements in ArrayList in the forward direction by calling hasNext() method in a while loop. If hasNext() returns true, then next() gives us the next element.
• Loop stops when hasNext() returns false, i.e. ListIterator points to the end of the list.
• With the same ListIterator, we have cycled over the elements in reverse direction by calling hasPrevious() in a while loop. If hasPrevious() returns true, previous() gives us the previous element.
• Loop stops when hasPrevious() returns false, i.e. ListIterator points to the beginning of the list.

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