Question

Create a nested inner class inside a file called LinkedList.java called the ListIterator.

ListIterator: This public class is nested within an LinkedList data structure and allows us to traverse the elements in any collection, access any data element and remove any data elements of the collection. It also adds the functionality to move forward and backward, which is new functionality. We are going to build this data structure from scratch.

1. Instance variables (fields) are:
   mPrevious (Node) – the previous node visited in the LinkedList.  
   mNext (Node) – the next node about to be visited in the LinkedList.
   mCursor (int) - the position of the iterator in the LinkedList (starts at 0)

ListIteratorDemo: Create a new ListIteratorDemo.java file and in the public static void main(String[ ] args) method, perform the following:

1. In the main method, create a new LinkedList to store Strings: LinkedList<String> coursesList = new LinkedList<>()
2. Create a new ListIterator list on the coursesList.
3. While the list iterator hasNext, call the next() method and print out the Element. This will traverse the LinkedList from head to tail.
4. Now, we'll traverse backwards. After step 4, the list iterator is at the tail, so while the list iterator hasPrevious, call the previous() method and print out the Element. This will traverse the LinkedList from tail to head.

Answer 1

LinkedList class.

 import java.util.NoSuchElementException; /** * Sample implementation of LinkedList. */ public class LinkedList<E> { private Node head; private Node last; private int size; public LinkedList() { head = null; last = null; size = 0; } public void add(E val) { if (head == null) { head = new Node(val); last = head; } else { Node newNode = new Node(val); last.next = newNode; newNode.prev = last; last = newNode; } size++; } public ListIterator iterator() { return new ListIterator(); } public class ListIterator { ListIterator() { mPrevious = null; mNext = head; mCursor = 0; } private Node mPrevious; private Node mNext; private int mCursor; public boolean hasNext() { return mNext != null; } public boolean hasPrevious() { return mPrevious != null; } public Node next() { if (!hasNext()) { throw new NoSuchElementException(); } // return mNext and make mNext to next of that. Node next = mNext; mNext = mNext.next; mPrevious = next; mCursor++; return next; } public Node previous() { if (!hasPrevious()) { throw new NoSuchElementException(); } Node prev = mPrevious; mPrevious = mPrevious.prev; mNext = prev; mCursor--; return prev; } } static class Node<E> { Node next; Node prev; E val; Node(E val) { this.val = val; } } } 

ListIteratorDemo class

 public class ListIteratorDemo { public static void main(String [] args) { LinkedList<String> coursesList = new LinkedList<>(); coursesList.add("Course1"); coursesList.add("Course2"); coursesList.add("Course3"); coursesList.add("Course4"); coursesList.add("Course5"); LinkedList.ListIterator iterator = coursesList.iterator(); // traverse it forward. System.out.println("Forward: "); while (iterator.hasNext()) { System.out.println(iterator.next().val); } // traverse it backward. System.out.println("Previous: "); while (iterator.hasPrevious()) { System.out.println(iterator.previous().val); } } } 

Screenshot of the code working with output.

public static void main(String [] args) { LinkedList<String> coursesList = new Linkedlist>(); coursesList.add("Course1"); coursesList.add("Course2"); coursesList.add("Course3"); coursesList.add("Course4"); coursesList.add("Course5"); LinkedList.ListIterator iterator = coursesList. iterator(); // traverse it forward. ListIterator Demo > main() | ListIterator Demo X | /Library/Java/JavaVirtual Machines/jdk1.8.0_231.jdk/Contents/Home/bin/java ... Forward: Course1 Course2 Course3 Course4 Course5 Previous: Course5 Course4 Course3 Course2 Course1