//DoubleNode class

class DoubleNode { String str; DoubleNode prev, next; DoubleNode(String str) { this.str = str; prev = null; next = null; } String getStr() { return str; } DoubleNode getNext() { return next; } void setNext(DoubleNode next) { this.next = next; } DoubleNode getPrev() { return prev; } void setPrev(DoubleNode prev) { this.prev = prev; } }

//LinkList class class DoublyLinkedList { DoubleNode head, tail; DoublyLinkedList() { head = null; tail = null; } DoubleNode getHead() { return head; } DoubleNode getTail() { return tail; } void addFirst(String s) { DoubleNode node = new DoubleNode(s); if (head == null) tail = node; else head.setPrev(node); node.setNext(head); head = node; } void addLast(String s) { DoubleNode node = new DoubleNode(s); if (head == null) head = node; else tail.setNext(node); node.setPrev(tail); tail = node; } void printList() { System.out.print("The list: "); DoubleNode node = head; while (node != null) { System.out.print(node.getStr() + " "); node = node.getNext(); } System.out.println(); }

void printListReverse() { System.out.print("The list in reverse: "); DoubleNode node = tail; while (node != null) { System.out.print(node.getStr() + " "); node = node.getPrev(); } System.out.println(); } DoubleNode search(String s) { DoubleNode node = head; while (node != null && !node.getStr().equals(s)) { node = node.getNext(); } return node; } DoubleNode getElement(int index) { DoubleNode node = head; int pos = 0; while (node != null && pos < index) { node = node.getNext(); pos++; } return node; } void insert(DoubleNode node, String newElem) // insert after that node // if node=null --> insert as first element { DoubleNode newNode = new DoubleNode(newElem); if (node == null) addFirst(newElem); else { DoubleNode nextNode = node.getNext(); newNode.setNext(nextNode); node.setNext(newNode); newNode.setPrev(node); if (nextNode == null) // or node == tail tail = newNode; else nextNode.setPrev(newNode); } } void remove(DoubleNode node) { DoubleNode prev = node.getPrev(); DoubleNode next = node.getNext(); if (head == tail) // one element { head = null; tail = null; } else if (head == node) // remove first element { next.setPrev(null); head = next; } else if (tail == node) // remove last element { prev.setNext(null); tail = prev; } else { prev.setNext(next); next.setPrev(prev); } } // Creating swapping method public void swap(DoubleNode x,DoubleNode y) { // Declaring temp node and swapping DoubleNode previousNodeX = x.getPrev(); DoubleNode nextNodeX = x.getNext(); DoubleNode previousNodeY = y.getPrev(); DoubleNode nextNodeY = y.getNext(); // handling null pointer exception if((x.getPrev()== null)&& (y.getNext()== null)&&(x.getNext() != y)) { y.setPrev(previousNodeX); y.setNext(nextNodeX); nextNodeX.setPrev(y); x.setPrev(previousNodeY); x.setNext(nextNodeY); previousNodeY.setNext(x); head = y; tail = x; } else if ((x.getPrev()== null) && (x.getNext() == y)) { y.setPrev(previousNodeX); nextNodeY.setPrev(x); x.setNext(nextNodeY); x.setPrev(y); y.setNext(x); head = y; } else if ((y.getNext()== null) && (x.getNext() == y)) { previousNodeX.setNext(y); y.setPrev(previousNodeX); x.setNext(nextNodeY); x.setPrev(y); y.setNext(x); tail = x; } else if ((y.getNext()== null) && (x.getNext() != y)) { y.setPrev(previousNodeX); y.setNext(nextNodeX); previousNodeX.setNext(y); nextNodeX.setPrev(y); x.setPrev(previousNodeY); x.setNext(nextNodeY); previousNodeY.setNext(x); tail = x; } else if ((x.getPrev()== null) && (x.getNext() != y)) { y.setPrev(previousNodeX); y.setNext(nextNodeX); nextNodeX.setPrev(y); x.setPrev(previousNodeY); x.setNext(nextNodeY); nextNodeY.setPrev(x); previousNodeY.setNext(x); head = y; } else if ((x.getPrev()!= null)&& (y.getNext()!= null)&& (x.getNext() == y) ) { previousNodeX.setNext(y); y.setPrev(previousNodeX); nextNodeY.setPrev(x); x.setNext(nextNodeY); x.setPrev(y); y.setNext(x); } else if ((x.getPrev()!= null)&& (y.getNext()!= null)&& (x.getNext() != y)) { y.setPrev(previousNodeX); y.setNext(nextNodeX); nextNodeX.setPrev(y); previousNodeX.setNext(y); x.setPrev(previousNodeY); x.setNext(nextNodeY); nextNodeY.setPrev(x); previousNodeY.setNext(x); } } //Creating the sort method public void sort() { //DoubleNode newHead = new DoubleNode(head.getStr()); DoubleNode pointer = head.next; pointer.setPrev(head.next.getPrev()); pointer.setNext(head.next.getNext()); while(pointer != null) { DoubleNode innerPointer = head; //System.out.println(head.str); innerPointer.setPrev(head.getPrev()); innerPointer.setNext(head.getNext()); DoubleNode next = pointer.getNext();

if((innerPointer.getStr()).compareTo((pointer.getStr()))>0) { this.swap(innerPointer,pointer ); } else { while((innerPointer.getStr()).equalsIgnoreCase(pointer.getStr())== false ) { System.out.println("inner pointer "+innerPointer.getStr()); System.out.println("pointer "+pointer.getStr()); if((innerPointer.getStr()).compareTo((pointer.getStr()))>0) { this.swap(innerPointer, pointer); } innerPointer = innerPointer.next; // System.out.println(innerPointer.getStr()); } //System.out.println(pointer.getStr()); } pointer = next; } } }

//Main class calling sort

public class Assignment6Q1 { public static void main(String[] args) { DoublyLinkedList list = new DoublyLinkedList(); list.addFirst("Orange"); list.addFirst("Grape"); list.addFirst("Banana"); list.addFirst("Kiwi"); list.addFirst("Apple"); list.addFirst("Pear"); list.printList(); list.swap(list.head, list.head.next); list.sort(); list.printList(); list.printListReverse(); } }

Please help with the Null pointer error, I cant just change the data for this project, I actually have to move the nodes.Here I am trying to write an insertion sort method but for some reason my code is giving me nullPointerException from :- at assignment.pkg6q1.DoublyLinkedList.swap(DoublyLinkedList.java:219)

at assignment.pkg6q1.DoublyLinkedList.sort(DoublyLinkedList.java:257)

at assignment.pkg6q1.Assignment6Q1.main(Assignment6Q1.java:18)

BUILD FAILED (total time: 0 seconds)