How do I fix it in java? LinkedList.java: /** * Defines a doubly-linked list class * @author * @author */ import java.util.NoSuchElementException; public class LinkedList { private class Node { private T data; private Node next; private Node prev; public Node(T data) { this.data = data; this.next = null; this.prev = null; } } private int length; private Node first; private Node last; private Node iterator; /**** CONSTRUCTORS ****/ /** * Instantiates a new LinkedList with default values * @postcondition */ public LinkedList() { first = null; last = null; iterator = null; length = 0; } /** * Converts the given array into a LinkedList * @param array the array of values to insert into this LinkedList * @postcondition */ public LinkedList(T[] array) { } /** * Instantiates a new LinkedList by copying another List * @param original the LinkedList to copy * @postcondition a new List object, which is an identical, * but separate, copy of the LinkedList original */ public LinkedList(LinkedList original) { } /**** ACCESSORS ****/ public T getFirst() throws NoSuchElementException { if (isEmpty()){ throw new NoSuchElementException("The list is empty"); } return first.data; } public T getLast() throws NoSuchElementException { if (isEmpty()){ throw new NoSuchElementException("The list is empty"); } return last.data; } /** * Returns the data stored in the iterator node * @precondition * @return the data stored in the iterator node * @throw NullPointerException */ public T getIterator() throws NullPointerException { if (iterator != null){ return iterator.data; }else{ throw new NullPointerException("Iterator is off the end opf the list."); } } /** * Returns the current length of the LinkedList * @return the length of the LinkedList from 0 to n */ public int getLength() { return length; } /** * Returns whether the LinkedList is currently empty * @return whether the LinkedList is empty */ public boolean isEmpty() { return length == 0; } /** * Returns whether the iterator is offEnd, i.e. null * @return whether the iterator is null */ public boolean offEnd() { return iterator == null; } /**** MUTATORS ****/ public void addFirst(T data) { Node newNode = new Node(data); if(isEmpty()){ first = newNode; last = newNode; } else{ newNode.next = first; first.prev = newNode; first = newNode; } length++; } public void addLast(T data) { Node newNode = new Node(data); if(isEmpty()){ first = newNode; last = newNode; } else{ last.next = newNode; newNode.prev = last; last = newNode; } length++; } /** * Inserts a new element after the iterator * @param data the data to insert * @precondition * @throws NullPointerException */ public void addIterator(T data) throws NullPointerException{ if(iterator != null){ Node newNode = new Node(data); newNode.next = iterator.next; iterator.next = newNode; if (iterator == first){ first = newNode; } }else{ throw new NullPointerException("Iterator is off the end opf the list."); } } / public void removeFirst() throws NoSuchElementException { if(is.

1. How do I fix it in java?
LinkedList.java:
/**
* Defines a doubly-linked list class
* @author
* @author
*/
import java.util.NoSuchElementException;
public class LinkedList {
private class Node {
private T data;
private Node next;
private Node prev;
public Node(T data) {
this.data = data;
this.next = null;
this.prev = null;
}
}
private int length;
private Node first;
private Node last;
private Node iterator;
/**** CONSTRUCTORS ****/
/**
* Instantiates a new LinkedList with default values
* @postcondition
*/
public LinkedList() {
first = null;
last = null;
iterator = null;
length = 0;

2. }
/**
* Converts the given array into a LinkedList
* @param array the array of values to insert into this LinkedList
* @postcondition
*/
public LinkedList(T[] array) {
}
/**
* Instantiates a new LinkedList by copying another List
* @param original the LinkedList to copy
* @postcondition a new List object, which is an identical,
* but separate, copy of the LinkedList original
*/
public LinkedList(LinkedList original) {
}
/**** ACCESSORS ****/
public T getFirst() throws NoSuchElementException {
if (isEmpty()){
throw new NoSuchElementException("The list is empty");
}
return first.data;
}
public T getLast() throws NoSuchElementException {
if (isEmpty()){
throw new NoSuchElementException("The list is empty");
}
return last.data;
}

3. /**
* Returns the data stored in the iterator node
* @precondition
* @return the data stored in the iterator node
* @throw NullPointerException
*/
public T getIterator() throws NullPointerException {
if (iterator != null){
return iterator.data;
}else{
throw new NullPointerException("Iterator is off the end opf the list.");
}
}
/**
* Returns the current length of the LinkedList
* @return the length of the LinkedList from 0 to n
*/
public int getLength() {
return length;
}
/**
* Returns whether the LinkedList is currently empty
* @return whether the LinkedList is empty
*/
public boolean isEmpty() {
return length == 0;
}
/**
* Returns whether the iterator is offEnd, i.e. null
* @return whether the iterator is null
*/

4. public boolean offEnd() {
return iterator == null;
}
/**** MUTATORS ****/
public void addFirst(T data) {
Node newNode = new Node(data);
if(isEmpty()){
first = newNode;
last = newNode;
}
else{
newNode.next = first;
first.prev = newNode;
first = newNode;
}
length++;
}
public void addLast(T data) {
Node newNode = new Node(data);
if(isEmpty()){
first = newNode;
last = newNode;
}
else{
last.next = newNode;

5. newNode.prev = last;
last = newNode;
}
length++;
}
/**
* Inserts a new element after the iterator
* @param data the data to insert
* @precondition
* @throws NullPointerException
*/
public void addIterator(T data) throws NullPointerException{
if(iterator != null){
Node newNode = new Node(data);
newNode.next = iterator.next;
iterator.next = newNode;
if (iterator == first){
first = newNode;
}
}else{
throw new NullPointerException("Iterator is off the end opf the list.");
}
}
/
public void removeFirst() throws NoSuchElementException {
if(isEmpty()){
throw new NoSuchElementException("The list is empty");
}

6. if(length == 1){
first = null;
last = null;
iterator = null;
}
else{
if(iterator == first){
iterator = null;
}
first = first.next;
first.prev = null;
}
length--;
}
public void removeLast() throws NoSuchElementException {
if(isEmpty()){
throw new NoSuchElementException("The list is empty");
}
if(length == 1){
first = null;
last = null;
iterator = null;
}
else{
if(iterator == last){
iterator = null;
}
last = last.prev;
last.next = null;
}
length--;
}

7. /**
* removes the element referenced by the iterator
* @precondition
* @postcondition
* @throws NullPointerException
*/
public void removeIterator() throws NullPointerException {
if(iterator != null){
if(iterator == first){
first = first.next;
}else{
Node prev = first;
while(prev.next != iterator){
prev = prev.next;
}
prev.next = iterator.next;
if (iterator == last){
last = prev;
}
}
iterator = null;
}else {
throw new NullPointerException("Iterator is off the end opf the list.");
}
}
/**

8. * places the iterator at the first node
* @postcondition
*/
public void positionIterator(){
iterator = first;
}
/**
* Moves the iterator one node towards the last
* @precondition
* @postcondition
* @throws NullPointerException
*/
public void advanceIterator() throws NullPointerException {
if (!offEnd()){
iterator = iterator.next;
}else {
throw new NullPointerException("Iterator is off the end opf the list.");
}
}
/**
* Moves the iterator one node towards the first
* @precondition
* @postcondition
* @throws NullPointerException
*/
public void reverseIterator() throws NullPointerException {
if(iterator != first && iterator != null){
Node prev = first;

9. while (prev.next != iterator){
prev = prev.next;
}
iterator = prev;
}
}
public void clear() {
first = null;
last = null;
iterator = null;
length = 0;
}
public String toString() {
StringBuilder result = new StringBuilder();
Node temp = first;
while (temp != null){
result.append(temp.data + " ");
temp = temp.next;
}
return result.toString() + "n";
}

10. @SuppressWarnings("unchecked") //good practice to remove warning here
@Override
public boolean equals(Object obj) {
return false;
}
/**CHALLENGE METHODS*/
public void spinList(int numMoves) throws IllegalArgumentException{
}
public LinkedList altLists(LinkedList list) {
return null;
}
}
.
LabProgram.java
java.lang.NullPointerException
t java.util.Scanner; c class LabProgram { ublic static void main(String[] args) { LabProgram lab
= new LabProgram(); // Make and display list LinkedList Integer > list = new LinkedList
langlerangle() ; for (int i=1;i<4;i++ ) { list.addLast (i); } System.out.print("Created list: " +
list.toString()); // toString() has In System.out.println("list.offEnd(): " + list.offEnd());
System.out.println("list.positionIterator()"); list.positionIterator();
System.out.println("list.offEnd(): " + list.offEnd()); System.out.println("list.getIterator(): " +
list.getIterator()); System.out.println("list.advanceIterator()"); list.advanceIterator();
System.out.println("list.getIterator(): " + list.getIterator());
System.out.println("list.advanceIterator()"); list.advanceIterator();
System.out.println("list.getIterator(): " + list.getIterator()); System.out.println("list.
reverseIterator()"); list.reverseIterator(); System.out.println("list.getIterator(): " +
list.getIterator()); System.out.println("list.addIterator(42)"); list.addIterator (42);
System.out.println("list.getIterator(): " + list.getIterator()); System.out.print("list.tostring(): " +
list.tostring()); System.out.println("list.advanceIterator()"); list.advanceIterator();
System.out.println("list.advanceIterator()"); list.advanceIterator();
System.out.println("list.addIterator(99)"); list.addIterator(99); System.out.print("list.toString(): "

11. + list.toString()); System.out.println("list.removeIterator()"); list. removeIterator();
System.out.print("list.toString(): " + list.toString()); System.out.println("list.offEnd(): " +
list.offEnd()); System.out.println("list.positionIterator()"); list.positionIterator();
System.out.println("list.removeIterator()"); list.removeIterator();
System.out.println("list.offEnd(): " + list.offEnd()); System.out.print("list.tostring(): " +
list.toString()); System.out.println("list.positionIterator()"); list.positionIterator();
System.out.println("list.advanceIterator()"); list.advanceIterator();
System.out.println("list.advanceIterator()"); list.advanceIterator();
System.out.println("list.removeIterator()"); list.removeIterator();
System.out.print("list.toString(): " + list.toString());